Over five-sixths of all food stamp households would have their benefits cut or wiped out under President Reagan's budget proposals, according to the Congressional Budget Office.

About an eighth of the households--roughly 3 million people--would lose their benefits entirely.

These cuts would come on top of last year's, which took about a million people off the rolls.

The CBO figures, released yesterday by Sen. Patrick J. Leahy (Vt.), senior Democrat on the Senate nutrition subcommittee, indicate that the cuts would particularly hurt the working poor.

All told, working poor families would lose $2 out of every $5 they now receive; the average yearly loss per family would be $684.

Leahy said such cuts would amount to "encouraging people to stop working and go on welfare," just the reverse of the president's professed goals.

In elderly and disabled families, Leahy said, $1 of every $4 in present benefits would be lost; the average yearly loss per family would be $192.

In his cold state of Vermont, he said, many elderly persons "are barely making it now," and further erosion of their stamp benefits would make them face "a daily choice: eat or heat."

In his budget message, Reagan asked that food stamp outlays in fiscal 1983 be cut from $11.8 billion to $9.5 billion by various means, including counting fuel-aid as income (the higher a household's income, the fewer stamps it is eligible for), cutting stamps 35 cents for every extra dollar of income instead of 30 cents as now, and making no further allowance for work expenses in computing income. The last provision will hit the working poor most heavily. The administration said its proposals would target aid more sharply on the truly needy. About 7.5 million families (21.8 million people) now get food stamps.

Leahy gave these examples prepared by his staff of how the proposed changes would work:

An elderly couple living on $425 a month ($5,100 a year) in Social Security benefits now is eligible for $312 annually in food stamps. Under the administration proposal, their food stamps would be cut to $108 a year. If the couple also was receiving $30 a month in fuel aid, their food stamp benefits would be wiped out altogether.

If one family were earning $5,000 a year in take-home pay by working, and another were getting $5,000 in welfare, the working family would end up with $300 to $400 less in food stamps than the non-working family.

The CBO study estimated that about 12 percent of all the households now on food stamps would lose them altogether under the adminstration proposals, and another 73 percent would suffer reductions, so all told 85 percent would be cut or wiped out. Among the elderly alone, 26 percent would lose benefits altogether and another 66 percent would have them cut, so 92 percent all told would be affected. Among the working families, 23 percent would lose all benefits and another 71 percent would suffer reductions, for a total of 94 percent affected.