The Fairfax County Board of Supervisors yesterday unanimously approved a $5.1 million plan to replace 148 aging public school buses next year, but warned school officials it may retract its funding if the new buses do not have seat belts.

The School Board voted last Thursday to ask the supervisors for enough money over the next two years to replace 362 buses built before federal safety standards for such vehicles took effect in 1977. Thirty-three buses already are included in the School Board's tentative 1987 budget. The additional money would bring the total to 181, with the remaining buses to be bought in 1988.

The schools have just begun a three-month test of the effectiveness of seat belts on school buses after several recent accidents. The supervisors appropriated $100,000 last year to buy 70 buses equipped with seat belts for the test.

Supervisor Thomas M. Davis III (R-Mason), who said he was reluctant to give the school system $5.1 million without a public hearing, told Deputy School Superintendent John P. Hess that he might not support funding the bus replacement plan if the seat belts did not improve children's safety in the vehicles.

"I hate to appropriate all this money without being assured they get seat belts," Davis said. "If we don't go with seat belts, I want to know I can withdraw the funds ."

Hess said it was too early to draw any conclusions from the seat belt test. Final results are expected in April, which is also when the supervisors will hold county budget hearings.

Supervisor T. Farrell Egge (R-Mount Vernon) abstained from the vote. Supervisor Joseph Alexander (D-Lee) was absent from the board meeting.