The Reagan administration has slightly eased proposed regulations that would expand use of merit pay in the lower reaches of the federal service and reduce the protections senior workers now have against reductions in force.

The easing was prompted in part by fear at the Office of Personnel Management that an alternative merit pay bill now in Congress could lead for the first time to collective bargaining by federal employe unions on wage issues.

The new proposed rules, published in yesterday's Federal Register, are revisions of tougher ones OPM put out with much fanfare March 30 and later threatened to put in effect over strong objections from federal employe unions and Congress.

OPM officials said yesterday that they decided to revise the rules in hopes they will be more acceptable to Congress and therefore eliminate any need for a compromise bill drafted by Sen. Ted Stevens (R-Alaska).

OPM Director Donald J. Devine and Stevens, chairman of a Senate Governmental Affairs subcommittee, began work on the compromise in May after Stevens said he would block OPM from implementing its March proposal. They reached an impasse on that compromise bill a few weeks ago, Pat Korten, an OPM spokesman, said yesterday.

"Part of his proposal is totally unacceptable to us, so we had to withdraw . . . and move ahead on our own with regulatory action," Korten said.

Stevens' bill would prevent the OPM from expanding the government's current merit pay system to an estimated 1.4 million mid- and lower-level federal employes until a number of "merit pay experiments" were conducted.

These experiments, which would test various methods of merit pay, would be conducted with up to 150,000 employes at several federal agencies and would last up to four years, Stevens said.

The OPM "vehemently opposes" the testing, Korten said yesterday, because Stevens' bill would allow federal employe unions a role in deciding such questions as how the tests would be conducted and how pay increases would be awarded.

"This would be an enormous, enormous change," Korten said. "It would open the door for the first time to negotiations over pay."

Stevens had scheduled markup of his bill yesterday, but canceled it after the OPM published its revised rules. The agency will receive comments about its proposal until mid-August, meaning the new rules would not go into effect until fall at the earliest if they are not blocked by Congress. Stevens said he would study the OPM's action before deciding what to do.

While the OPM has tempered some of its more controversial proposals, the new rules still would end the virtually automatic within-grade raises most federal workers now receive.

Workers in the first six steps of a grade would get within-grade raises every year or two only if they received a rating of "fully successful" under a new five-tier rating system the OPM has proposed. More senior workers in the seventh through 10th steps of the grade would need a higher rating of either "exceeds fully successful" or "outstanding" to get the raises.

The OPM, however, has dropped a controversial plan that would have set new intervals between promotions.

The OPM also had proposed that future reductions in force be based on an employe's last performance rating rather than on seniority.

The OPM said this would help women and minorities, who often lack seniority and are the first fired under current seniority rules, but opponents said they feared it would give political appointees power to punish or fire uncooperative career workers. The revised rules say an employe's seniority and last three job performance ratings will be taken into account during layoffs.