The federal government would kick in $1 for every $2 its employes invest -- up to 8 percent of salary -- in tax-sheltered investment programs under a new Civil Service pension plan being floated by Sen. Ted Stevens (R-Alaska).

Stevens' plan, being checked out by the administration and federal worker groups, would authorize employes to shelter up to 16 percent of their income in so-called 401(k) programs, with Uncle Sam putting in an additional 8 percent. Employes would get the supplemental benefits when they retire as part of a package that includes Social Security and modified Civil Service benefits.

Stevens' retirement plan, which must be approved by Congress and the White House, is based on the tax code and is intended to cover federal and postal workers hired since last January.

The 2.6 million civil servants already covered by the regular federal retirement system could remain where they are, or buy into the Stevens plan at their option.

The regular Civil Service retirement system bases annuities on the employes' service time and salary. Federal workers are not covered by Social Security as part of their U.S. jobs, nor are they eligible to participate in the 401(k) investment plans that are available to many private-sector workers.

Stevens' plan, which probably will be modified before it is introduced as legislation early next year, would create a three-tier retirement system that would include a modified Civil Service retirement plan (to which employes would contribute nothing), Social Security, and the optional savings plan in the form of tax-deferred 401(k) investments.

Retirement benefits to employes under the new system would come from all three sources: Civil Service, Social Security and their own investments.

Federal workers hired before last January could remain in the present retirement plan or come into the new system. Those who remained in the current retirement program would continue to contribute 7 percent of salary into the retirement fund and pay the 1.3 percent Medicare portion of Social Security. Workers under the new plan would not contribute anything for Civil Service retirement benefits (the government would pick up the entire tax) but would pay the 7.05 percent Social Security tax.

The Office of Personnel Management has not objected to the 401(k) concept. But some Treasury Department officials think the amount employes would be allowed to shelter is excessive and runs counter to tax-reform proposals.

It will be months before any new retirement plan begins to take shape. The Congressional Research Service is preparing to issue a report on retirement systems, and the House Post Office-Civil Service Committee is also doing work on federal fringe benefits.

What is certain is that Congress next year must come up with a retirement plan for post-1984 hires and, as of now, the only model it has is the Stevens plan.