MELUN, France - Even by the standards of affluent American wheat farmers, the Heilbronn family is prospering.

Luxuriant flower beds bursting with the color of late fall blossoms line the driveway up to a stately manor house set apart from a complex of sturdy barns. Didier Heilbronn, dressed informally in knickers and open necked shirt, leads his Sunday visitors to his own separate house. Excellent wine, cheese and salad appear and Heilbronn, his wife and visitors settle in for conversation that skips easily from international economics to the latest gossip about Paris society. Afterwards, Heilbronn is ready to join his father and the rest of the family from the main house for some partridge shooting.

The Heilbronns are farmers - but clearly no ordinary farmers.Their farm, in a flat region of Brie, 25 miles southeast of Paris, is located in one of the world's richest agricultural regions. Last year, the Heilbronns raised 250 acres of wheat, 175 acres of sugar beets and some 25 acres of corn - a large operation by French standards.

As Didier Heilbronn readily concedes, "In this part of the country, the big grain farmer has had very little to lose." Soil, weather and location play an important part in that story of prosperity. But beyond those attributes of nature and geography, the farmers in this region have an additional windfall advantage. They are among the largest beneficiaries of a controversial European agricultural policy that makes it virtually impossible for big French grain producers to do anything but earn large profits.

That system, which will be 10 years old in 1977, was tailored to safeguard the incomes of hundreds of thousands of small, marginal European farmers, especially French farmers, and to slow the exodus of rural people to crowded, overburdened cities.

Indirectly, U.S. critics say, it has cost American grain farmers billions of dollars in lost exports by encouraging subsidized French farmers to produce as much as they can.

The benefits to France and its 1.5 million farms, on the other hand, have been substantial. Increments to Power

THE POLICY came into being as a swap between France and West Germany. France would be able to sell its farm surpluses freely to other European countries, while West Germany could market its industrial products freely in France.

Today, agriculture is still a major economic underpinning of French prosperity. Behind the protectionist West European agricultural policies. France has emerged as the world's sixth largest wheat producer and the third or fourth largest wheat exporter, trailing only the United States and Canada in some years. In addition, production of corn for animal feed, almost unkown in Western Europe 15 years ago, has spread rapidly in a belt from southwest France to Belgium, and the effect has been to deprive American corn exporters of an important traditional market.

The subsidies to farmers come in th form of high price supports for wheat, corn and sugar beets. The system is buttressed by extremely high trade barriers that limit the volume of cheaper American and other foreign grain entering Western Europe.

The closed system shields European farmers from the ups and downs of the world economy. This season, while American wheat farmers felt the impact of a global wheat surplus that pushed prices down to $2.50 a bushel in October, the Heilbronns averaged $4.20 a bushel for their crop.

Although the drought reduced the amount of wheat produced, the Heilbronn farm still yielded 40 bushels an acre, well above the American average. Stocks of wheat are accumulating rapidly all over the world, but French wheat farmers are still encouraged to plant as much as they can this year.

The subsidy policies, applied equally to all farmers regardless of the size of their holdings, have bestowed enormous benefits on the large, effective grain farms around Paris. Because of the high guaranteed prices, French grain men use more hybrid seed and douse the land with more chemical fertilizer than U.S. grain farmers.

The high returns have, in turn, pushed land values in the region to well over $1,000 an acre. The Heilbronns' neighbor, one of the largest corn farmers in France, has more than 500 acres in corn alone. Rationale and Irony

AMONG critics of the Common Market's European agricultural system, however, the lavish prosperity of France's large grain farmers is often cited as the symbol of a misguided policy.

Agriculture is perhaps the most politically explosive issue in Western Europe today - a subject that can still send bands of angry farmers into the streets and prod usually aloof officials into genuine furies.

Although the number of persons on farms has steadily declined since World War II, the population employed in agricultural is still slightly more than 10 per cent in the nine Common Market countries, compared with 4 per cent in the United States.

And in several countries whose governments have slim parliamentary majorities, agrarian lobbies hold the ballance of power.

In many respects, Western Europe epitomizes a contemporary irony: in wealthy, industrial regions, farm blocs tend to be much more powerful than they are in poorer, agricultural societies.

In France, annual agricultural exports - mostly wine, dairy products and grain - are valued at $8.9 billion, a fifth of the nation's total exports. Any tinkering with the present subsidy policy could threaten the powerful farm interests.

The policy frequently puts France at odds with the United States, not because the two countries have different interests, but because they face similar challenges. In both countries, the top farm problem is disposing of surpluses created by the combination of sharply increased productivity and domestic political requirements.

Europe is still the largest U.S. agricultural market. Soybeans, a basic animal feed not grown widely in Europe, flow in freely. But West European trade barriers have limited U.S. wheat imports to 2 to 3 million metric tons a year, and officials at the U.S. Department of Agriculture complain bitterly that the Common Market has subsidized the region's flour exports so blatantly that they undersell American flour by $50 a ton in world markets.

"It's a senseless policy by any doctrinaire economic standards," said a senior American diplomat in Brussels, capital of the Common Market.

European farm policies after World War II have been dominated by a "siege mentality," says Phillip M. Raup, professor of agricultural economics at the University of Minnesota. After decades of food deficits, he contends, the Western Europeans have had trouble adjusting to an era of plentiful, and sometimes burdensome, food supplies.

Farm prices in West Europe often bear little resemblance to those in the rest of the world. For example, in October a European flour miller could have bought wheat for only $122 a ton on the world market. But he would have had to pay an $81 a ton duty on the wheat to bring the price up to the fixed, minimum level at which the grain is traded within the European community. So there are no advantages in buying the foreign wheat unless the miller needs a variety unavailable in Europe.

Among the "Eurocrats" in Brussels, nobody claims the policies have been either cheap or overly beneficial to consumers, who seem much less vociferous in defending their interests than Europe's farmers.

The price support payments cost $6 billion a year, and only part of that is covered by duties collected on imported grain. The individual governments also pay their farmers some $11 billion a year in welfare and insurance programs.

Despite surpluses, of butter and meat, food prices in Europe still tend to be higher than in the United States. In September, broiler chickens were selling for 53 cents a pound in Washington and 82 cents in Bonn, and a chuck roast cost $2.88 a pound in London compared with $1.22 in the U.S. capital. In other words, European consumers help support the high prices paid to farmers. Paying for Unity

THE SYSTEM'S supporters say the drawbacks are offset by many positive accomplishments.

The joint agricultural policy, known in Common Market bureaucratic jargon as the CAP, is one of the few concrete achievements of the movement for European unity. Dismanting it would therefore be a sharp blow to that unity.

The defenders add that, most important, the system has been an effective social policy tool, helping to control demographic forces that might otherwise have drained the countryside of people too rapidly.

Petrus J. Lardinois, the Common Market's agriculture minister, has praised the policy for "providing stability vital to our social fabric."

"There's no way back in the present economic situation," said a Brussels businessman. "We're frozen in place. For God's sake don't send any more farmers into our cities."

Were the Europeans to switch to free market farm policies similar to those in the United States, "thousands of farmers would be put out of business and perhaps three-quarters of our grain farmers would be destroyed," said Jean-Pierre Andrault of the French General Association of Wheat Producers in Paris.

Officials at the French Foreign Ministry also see geopolitical advantages in a policy that has enabled Europe to achieve near self-sufficiency in grain. They argue that countries which depend on others for most of their food lose some of their diplomatic flexibility. They argue that in a world subject to food shortages, as occurred in 1974, it makes little sense to limit national food production.

Yet European economic analysis concede that the farm system has become a bureaucratic monster, too complicated and unwieldy to serve its stated purposes effectively.

The free movement of farm goods inside the community - the basic prerequisite of the system - is being strained.

"It's acceptable to everybody because it's not really working," said a French government official. He meant that, if all the rules were being rigidly enforced, the governments might be at each other's throats more than they already are. In practice, there are more and more departures from the system's original intent.

Rapidly changing currency exchange rates have made a mockery of the plan to pay all European farmers the same price for their crops from northern Scotland to southern Sicily.

In fact, British farmers collected the equivalent of $130 a ton for wheat in October, while West German farmers were getting $188 a ton.

Farm prices are fixed annually by Brussels and pegged to the dollar, and farmers are paid according to that formula all through the year. But, as the West European currencies fluctuate widely against the dollar, these fixed farm prices quickly get out of line.

Rather than adjust the price of the farm products - a step that could have an immediate impact on food prices and inflation - Brussels has been compensating food importers and exporters with a complex system of internal duties and taxes.

The overall effect of the system is to subsidize the food imports and tax the food exports of countries with weak currencies, such as the British pound. Britain, which imports half its food, has been costing Brussels over $2 million a day in such compensations.

Because agricultural products are priced artificially low in England, foreign suppliers wouldn't sell the food there if Brussels did not pay them the added compensation. At the same time, British farmers are receiving less than they would if the prices for their farm products were not pegged to an old, outdated exchange rate.

The opposite is true in West Germany, where farm products are over-valued on the basis of current rates of exchange for the mark.

In both cases, the artificial system is in the interest of the governments in power. Bonn's weak coalition is afraid of angering its farm bloc and British leaders would rather face angry farmers than angry consumers, since an adjustment could raise food prices in Britain by 1.2 per cent.

So explosive are these seemingly technical matters that the Irish agriculture minister, in a rage, stormed out of a Common Market meeting in October when Britain fought an adjustment of the subsidy system.

The fear of some Europeans is that they may wake up some morning and discover that the fragile fabric of postwar European unity has been ripped apart by just such a dispute.

"If that happens," said a French official, "your farmers in America might be glad. But we would have lost the only thing we have put together as a region in 30 years."