A sharp drop in food production resulting from a freak combination of drought and frost is expected to swell the flow of poor Mexicans entering the United States illegally in search of jobs.

The chronically poor conditions in the Mexican countryside recently have caused an estimated 2 million Mexicans to sneak across the American border every year. But as a result of the dramatic crop failure here, U.S. bimmigration officials believe that this situation will be aggravated seriously in the coming months.

The extent of the agrarian crisis had been kept secret by the Mexican government until a few weeks ago, when it admitted that grain imports in 1980 would exceed 6 million tons, an almost 20 percent increase over 1979.

For Mexicans, from poor farmers to government planners, the recent crop failure will have far-reaching implications.

It means that just as Mexico is gaining a greater degree of financial and political independence from its oil wealth, it will have to increase its dependence on the U.S. market for basic food. Mexico makes up most of its shortages in corn, beans, wheat, grain, sorghum, oats and oil seeds by buying them in the United States.

The agricultural disaster also once again heats up the national debate about how to spend the oil income. This is the first year that the Mexican economy will show a net gain from its oil exports with an estimated $16 billion in oil profits.

Critics here, however, have long berated the government for making industrial devlopment the national priority while leaving the countryside largely to its own devices. Even without national disasters such as this season's, these critics say, this policy is leading Mexico to trade "oil for food" as a way of life.

The drought and frost of last fall have hit all staple foods: corn, which in the form of tortillas is the mainstay of most Mexicans' diet, dry beans, another favorite, and grain sorghum, the main ingredient of mixed animal feed.

According to the government, 2 million tons of corn -- 10 percent of the crop -- have been lost as well as 3 million tons of beans, which amounts to almost one-third of that harvest.

As a result, hundreds of thousands of poor farmers have lost their entire income for the year.

Most Mexican peasants grow enough food for their own needs and sell any small surplus to pay for the few extras they can afford in life. Most of Mexico's peasantry, in whose name the 1910 revolution was fought, still make do with unirrigated, often low-quality land.

The large commercial operations, which usually are irrigated, hardly have been affected. Moreover, most of them grow winter vegetables for the U.S. market and, because of their different planting cycles, they have escaped the frost.

The worst hit regions are central and northwest Mexico, lands where life is hard at the best of times and which traditionally have produced the greatest number of people who emigrate to the United States.

But the improverished southern mountain states of Guerrero and Oaxaca, largely inhabited by Indians who live practically outside the money economy, have suffered badly as well.

The Oaxaca governor, Gen. Eliseo Jimenez, described the situation in his state as "very dramatic, very tragic."

"More than 100,000 peasant families in Oaxaca have lost their entire harvest," Jimenez said.

Like other state governors, Jimenez has appealed for federal emergency aid. The government is hastily drawing up work programs in the countryside to employ the unskilled peasants in chores such as cleaning drainage ditches, doing road repairs and opening up new dirt roads.

The government's basic foodstuffs agency, Conasuto, is preparing to distribute greatly increased qualities of staple foods to the outlets across the country where it sells at subsidized prices.

But officials are aware that hundreds of thousands of Mexicans who live on substandard diets even in normal times may go hungry in coming months.

It is this serious prospect that has led U.S. immigration officials to conclude that pressure on the Mexican-U.S. border will increase sharply in the near future. Press reports from the north of Mexico indicate that the alien smuggling operations are already busier than this time last year.

"I have no doubt that more Mexicans will come," said a California-based lawyer, who defends migrants caught after entering the U.S. illegally."I have seen it before, when there is a drought or problems on the farms; more people cross over."

If this forecast is true, it will mean more work for the U.S. Border Patrol, which has been pressing for a sizeable increase in the number of officers, a request that was cut back sharply two months ago by the Office of Management and Budget.

"I know it will mean more work for all of us," the California lawyer said in a telephone interview, "It's kind of sad. Unlike the regulars, the people who come across only in an emergency are the ones who need it most. But they are also the least prepared and therefore the likeliest to be caught."