Federal workers and retirees, who now pay about 39 percent of the total premium for their health insurance, would get "free" coverage if they bought a basic, no-frills policy, under a voucher system plan the Reagan administration will send Congress shortly.

Under the voucher plan, which Congress ignored two years ago, the government would issue employes and retirees a check each year (one amount for single coverage or a larger voucher for family coverage) that could be used only to buy health insurance.

Currently, U.S. personnel pay anywhere from $300 to $1,500 a year in premiums, depending on the health plan they choose.

According to the Office of Personnel Management, the government pays an average 61 percent of the premiums. Postal employes, thanks to their union contract, pay less for health insurance because the U.S. Postal Service picks up a larger share of their premium.

The federal employe health benefits program is the nation's largest "company" health plan. It covers nearly half the people in the Washington area and provides protection for 10 million Americans ranging from ex-presidents and senators to government clerks and their families.

Administration officials believe the voucher plan would cut federal costs by encouraging employes to buy less expensive insurance and by encouraging insurance plans to come up with low-cost packages, the premiums of which would be covered in full by the vouchers. Workers who want more expensive policies, which often provide better coverage, would pay the difference in premiums.

Opponents of the voucher plan fear it would entice lower-paid workers and retirees into buying insurance packages that do not provide adequate coverage for them if they should be hit with major medical bills.

If the administration can make a good case for the voucher system it will get serious consideration from the next Congress, which is dedicated to reducing the deficit by cutting federal costs.