President Reagan asked Congress yesterday to change a law so that federal employes in the future would get smaller pay raises each year than their counterparts in the private sector.

He proposed that they be given 94 perecent of what they would otherwise get under the "comparability" principle that has prevailed -- in theory at least -- since 1962.

That is the most important of about $6 billion in federal pay and benefit cuts the president proposed for next year.

The president also proposed that Congress reduce future cost-of-living increases in federal civilian and military retirement benefits by figuring them once a year instead of twice as now -- something he had specifically promised not to propose as a candidate last fall. And proposed to eliminate about 43,000 nondefense federal jobs in the current fiscal year and another 19,000 in fiscal 1982, which begins Oct. 1.

Federal employe unions immediately pledged to fight Reagan's proposals, most of which would require legislation. On capitol Hill, members of Congress responsible for federal employment issues said that the outlook is uncertain, but that Reagan seems to have a better chance to get his way than did Jimmy Carter, who tried in vain to make many of the same changes.

One item Reagan does not need congressional authorization for is his plan to eliminate, by attrition, 62,000 nondefense jobs over the next two years. The elimination of those federal jobs would cut non-defense federal employment about 5 percent from current levels by the end of fiscal year 1982. The president indicated yesterday that he would leave it up to agency heads to achieve the reductions, and that he would eliminate his across-the-board freeze on federal hiring if agency heads can reduce employment levels by other methods.

Most of the changes Reagan proposed in the salary "comparability" law were similar to amendments proposed by Carter last year. The major difference -- and a major departure from the "comparability" principle implicit in current law -- is Reagan's plan to set federal pay scales at 94 percent of comparable private wages.

Federal white-collar workers are currently supposed to receive 100 percent of the comparable private scale; blue collar employes get from 94 percent to 116 percent of the comparable wage. To enforce this requirement, the government annually assesses private pay scales, and raises federal pay each October to maintain "comparability." In 1979, federal workers received a 7.1 percent increase. Last fall they got a 9.1 percent raise.

Carter's budget for fiscal 1982 had proposed a 5.5 percent increase next October. Reagan said yesterday he wants to hold the raise to 4.8 percent and save $3.8 billion from the amount projected in Carter's budget.

To get that saving, Reagan said he will send Congress legislation changing the way "comparability" is determined. The proposed law would compare federal and non-federal benefits as well as pay; since federal benefits frequently exceed those in the private sector, this would require a smaller upward adjustment in federal pay each year. Reagan also wants to include state and local remuneration in the "comparable" category, another change that would hold down federal raises. Further, the president wants to "localize" federal white-collar salaries, so that the government could pay less to its employes in cities less expensive than Washington.

Reagan said setting wages at 94 percent of the comparable non federal figure is far in light light of "those aspects of federal employment which make it more attractive than many comparably paid jobs "Among these aspects is a federal worker's right to change jobs and agencies without losing accrued benefits.

Carter also had proposed that federal pensions be adjusted only once each year to reflect increases in the cost of living; persions are currently adjustes twice yearly Estimates of the savings from that change range from about $750 million to $11 billion annually.

A Republican staff aide on the Senate Governmental Affairs Committee where Reagan's proposals will probably start their journey through Congress, said the pension adjustment bill has a good chance of passage but prospects for the "comparability" proposals are impossible to predict.