More than 1 million federal retirees would be eligible for health premium refunds of $18 to $400 apiece under a bipartisan bill the House approved yesterday.

The legislation, now before the Senate, also could lower insurance costs next year for civil servants and retirees by allowing the government to pay a bigger share of employe and retiree health premiums.

Eleven of the government's health plans are offering premium refunds to current policyholders. But because of the wording of the existing federal insurance law, only employes are now eligible for refunds.

The Reagan administration supports the change in the law, introduced by Rep. Mary Rose Oakar (D-Ohio), that would make retirees eligible for the same refunds.

Oakar's bill, cosponsored by Rep. Frank Wolf (R-Va.), also would make it possible for the government to pay more or all of the premium for employes and retirees who choose low-cost health insurance.

Under current law the government cannot pay more than 75 percent of the premium, which it does for postal employes. For most white-collar workers and retirees, the government pays about 61 percent of the premium. Eliminating the maximum federal contribution opens the door for the government to pay all or most of the premium charged by some plans.

Federal workers and retirees will be able to participate in an open enrollment period from Nov. 4 to Dec. 6, during which they may select the health plans they want for 1986. Premiums are already slated to drop an average of 6 percent, but the savings could be even greater if the Senate okays the Oakar bill. It is expected to do that.