The U.S. government would set up a new employe retirement plan offering much smaller basic annuities, but allowing employes to invest up to 16 percent of their salaries into tax-deferred 401(k) investment programs, under a plan Sen. Ted Stevens (R-Alaska) will introduce early next year.

Stevens' radical new retirement program would be mandatory for all federal and postal workers hired after last Jan. 1. The civil service retirement program that covers most of the government's 2.6 million workers would be unchanged, but those employes could voluntarily come into the new program.

Congress and the White House would have to approve Stevens' plan before it could be put into effect, beginning in 1986.

Here are the key points in Stevens' plan:

*Workers covered by it would not contribute any money to the civil service retirement fund. Their agencies would continue to make contributions. Retirees in the existing civil service program would continue to put 7 percent of their salaries into the fund. The Reagan administration -- in a separate action -- will ask Congress next year to raise the contribution rate to 9 percent.

*Because it would be a noncontributory retirement fund (from the employe standpoint), the formula for calculating benefits would be reduced, resulting in basic annuities under the new system that are about half those of benefits due employes under the existing civil service plan.

*Employes under the new system, and longtime workers who elect to join, could put up to 16 percent of their salaries into a tax-sheltered 401(k) program. That money would be invested in the private sector. Money put into a 401(k) account would not be taxable until the employe withdrew it.

*Workers under the new system would continue to pay the full Social Security tax (starting next year that will be 7.05 percent of salary on amounts up to $39,600) and get full benefits from Social Security at age 65, or reduced benefits at age 62. The civil service benefit would be reduced when employes started getting payments from Social Security. Federal employes who elected to remain in the existing system would pay only the 1.3 percent Medicare portion of Social Security.

*Pensions under the new system would be based on length of service and the highest five-year average salary of each worker. Under the current plan annuities are figured on time in government and the employe's highest three-year salary average.

*Under Stevens' plan, workers could retire on immediate annuities at age 62 with 10 years of service, or at age 55 with 30 years' service. Basic annuity, however, would be reduced 2 percent for each year the employe was under age 62 when he or she retired.

*Contributions to 401(k) plans would be matched, up to about 4 percent, by the government.

At retirement, employes under the proposed plan would be able to take their money from 401(k) accounts, roll it over into an Individual Retirement Account, or begin drawing it as an annuity supplement.

*Cost of living raises for persons retiring under the proposed new system would be 75 percent of the actual annual rise in living costs. Retirees under the current system get annual adjustments equal to 100 percent of the inflation rate.

Stevens' plan would not change the current civil service retirement program. But employes could join the new system voluntarily.

Insiders say Stevens' plan would be most attractive to employes who could take advantage of the tax-deferred investment features, but considerably less attractive to low-salaried workers.