For the Reaganite fiscal planners assigned to cut President Carter's final spending plan down to conservative size, there's a line item on page 402 of the new budget that looks like a particularly juicy target.

"Food stamp appropriation, current," it reads, and it shows that the government is spending $9.7 billion on food stamps this year and will spend $12.9 billion for the same program in fiscal 1982. That'sa 33 percent jump in one year, which is about normal for this line item. The food stamp budget has grown by 95 percentover the last three years. If you make a chart of the program's 17-year life, the graph looks like the side of the Empire State Building -- there are a few ledges, but mostly thedollar curve goes straight up.

Ronald Reagan and his advisers describe the food stamp program as a paradigm of the "welfare extravagance" and "uncontrolled spending" the president-elect has been attacking for two decades. But the program is also a classic illustration of the problems. Reagan can expect in cutting the fundamental and costly benefit programs that make up the bulk of the budget.

In dollar terms, handing out benefits is the government's No. 1 Function.

About 42 percent of Carter's new budget, or around $314 billion, will go for individual benefit payments. The government in Washington has become an enormous Robin Hood, shifting hundreds of billions of dollars from upper- and middle-class taxpayers to the poor, from younger workers to older pensioners.

On economic grounds, the best way to cut federal spending on individual benefits is to change the conditions that lead people to need benefits. For every 1 percent dropin unemployment rates, according to the Department of Agriculture, the annual food stamp budget drops by half-a-billiondollars. For every 1 percent drop in the inflation rate, the government also stands to save $1.2 billion in future benefit outlays.

But Reagan's economic thinkers now say theyexpect high umemployment and inflation to persist for the rest of this year, at least. So the budget-cutters will haveto look for help elsewhere.

Under the food stamp program, the Department of Agriculture sends stamps, or coupons -- in effect, a kind of currency that can be used only to buy food -- to the states, which distribute the stamps each monthto people who can prove their need. Stamps are the only major federal benefit paid out strictly on the basis of poverty; anyone poor enough can get stamps regardless of age, health, family status, work history or anything else.

About 22 million people -- roughly one of every 10 Americans -- will get food stamps this month. How many stamps depends on the recipient's financial status; the higher a person's income, the lower his benefit. The average allotment is about $41per month per person.

One evident way to cut the food stamp program is to cut the number of people eligible for stamps. This is hard to do, though, because all studies indicate that most of the people receiving the stamps really need them.

The last survey of food stamp recipients, taken in 1979, showed that 85 percent lived in households with gross annual incomes of $6,000 or less. Ninety percent had incomesbelow the amount the government describes as "poverty level." The survey -- based on audits by federal and state investigators and not on the recipients' own description of their status -- found few of those high-living welfare types some politicians complain about. It concluded that about one-half of 1 percent of food stamp recipients lived in households with annual incomes over $12,000. Most of those households had seven or more inhabitants.

The government determines whether a food stamp applicant is eligible each month by estimating his income for the month to come. One idea the Reagan people are contemplating is basing eligibility on an average of the previous three months' income. This proposal should eliminate some people who are only temporarily out of work, and thus save some money.

But state governments, which run the program day-to-day, say a change like that would be too hard to administer. When the Ford administration proposed the same change in 1976, there was so much clamor from state administrators that the idea was dropped. Most other proposals for changes in the eligibility rules are likely to generate the same kind of protest.

The words "waste" and "fraud" have accompanied the food stamp program almost sinceits inception, and some Republicans in Congress talk about eliminating both as a way to cut program costs. Even the program's greatest champions agree that some of the money is wrongly or fraudulently spent on people who are not really eligible; estimates range from small amounts to more than 10 percent of the total program cost.

But Reagan's administration will not be the first to try to eliminate waste from food stamp grants, and it is not clear that he will be able todo any better at this nebulous chore than his predecessors.

If it is tough to reduce the number of eligible beneficiaries, the adminstration might focus instead on the amount of money spent on each eligible person. At present, for example, the government figures that everyone needs the same amount of food each month. The new adminstration might try to reduce the amount given certain people, on the the theory that a retired grandmother needs less food each month than a growing teen-aged boy.

This idea -- also proposed and withdrawn by the Ford administration -- makes nutritional sense but causes political problems. The big losers under such a change would be two politically potent groups -- women and the elderly.

There is one final problem, too, facing a president who wants to cut food stamps or any other federal benefit. Each benefit program has two constituencies: Beneficiaries and vendors who sell goods to the beneficiaries. In the case of the food stamp, the vendor community is the nation's farmers.The senate Agriculture Committee reports that every $100 million spent on food stamps increases farm income by $36 million.