The Senate Civil Service subcommittee next week will report out a bill that would block the Reagan administration plan to set up a merit pay system for most of the government's million-plus white-collar workers.

Administration officials hope to change federal work rules next month so that pay raises and job security of employes would be based on performance rather than seniority.

Under the administration plan, within-grade raises (worth 3 percent) would be based on an employe's performance rating, not time in grade.

The rules change also would reverse the current reduction-in-force pecking order, giving employes with the best job ratings the greatest protection from RIF and devaluing seniority.

Just before it adjourned Congress ordered the Office of Personnel Management not to make any work-rules changes until mid-October. OPM officials believe that they can come up with a compromise that will meet the objections of members of Congress who fear the changes could be abused by bosses.

But a compromise plan before the subcommittee--opposed by the OPM--would limit the merit pay plan to a "demonstration" project involving only 150,000 employes. Under that plan, which would be tested for 3 1/2 years, unions could negotiate terms of RIF rules and merit pay guidelines.

The subcommittee is sure to clear the bill because it is chaired by Sen. Ted Stevens (R-Alaska), who wrote the bill. It is also likely to be okayed by the parent Governmental Affairs Committee although it could run into trouble on the Senate floor from members who fear it would give unions a foot in the door to bargain for wages. Reagan administration officials are busy lining up opposition to the bill in the Senate.

But they are being a little more diplomatic than usual because they need Stevens' help--he is the Senate's assistant majority leader--on more important matters unrelated to the civil service.

Federal unions support the Stevens plan, meaning it will have smooth sailing in the House if it can get off the Senate floor.

If the Stevens plan is shot down, opponents of OPM's much tougher proposal still think they can kill it in Congress. But OPM officials insist that the Stevens bill will fall on its face, and that they will be able to begin putting their merit pay plan into effect this year.