Federal health insurance premiums, due to drop an average of 6 percent next year, would be reduced even further under a budget proposal Democrats hope to push through the House Post Office-Civil Service Committee today.

The plan, part of the budget reconciliation process, calls for trimming employe premiums by raising the government contribution.

Congress has already approved an overall budget for the fiscal year that begins next month. Senate and House committees with jurisdiction over various programs are now recommending how goals set forth in the budget will be achieved.

The Post Office-Civil Service Committee has jurisdiction over federal pay and fringe benefits and its Democratic majority has long been at odds with virtually every program and proposal made by the president. Whatever the committee recommends must still be approved by Congress before final budget agreement is reached.

Committee Democrats want the budget to guarantee raises to U.S. employes -- who will get no increase next year -- of 5 percent in 1987 and 1988.

The proposed insurance changes could raise take-home pay of workers next year by cutting their health premiums. Agencies now pay 61 percent of the average premium for white-collar workers and retirees. Because of its union contract, the U.S. Postal Service pays 75 percent of its employes' premiums.

If committee Democrats get their way, the government share of health insurance premiums will rise, making it possible for the government to pay up to 80 or 90 percent of the insurance premium.

Some committee Republicans object to putting guaranteed federal raises in the budget. Congress, which approved the 1986 pay freeze long before President Reagan made it official, has indicated that U.S. workers may get a 3.8 percent raise in 1987 and a 4.7 percent raise in 1988, subject to change by the president.

If the committee authorizes the pay raises and the health premium changes, nothing will be official -- or legal -- unless approved by the full House and the Senate as part of the final budget package.