All new buses for the Fairfax County school system probably will come equipped with seat belts, county School Superintendent Robert R. Spillane said yesterday. Spillane has asked for 50 new school buses in his proposed 1986-87 budget.

The county, which has the nation's largest school bus fleet, ordered 70 new school buses with seat belts for its regular runs this semester and will evaluate whether they improve safety. Many school systems already use seat belts for handicapped students, but Fairfax is among the first to experiment with them on standard buses.

Spillane's budget includes $2.2 million for 50 additional buses, drivers and attendants. If money is available, he would like the county to spend nearly $2.7 million to replace 77 buses dating to 1973 and 1974.

Spillane said he expects to order new buses with seat belts "unless we find out something very significant about those school buses in the next three months."

Opinion is divided on whether the seat belts improve school bus safety, with parent groups and some officials saying they do, and the school bus industry and some federal experts saying they do not.

The seat belts cannot be installed in buses already on the road. They add $1,300 to the cost of new buses. Board of Supervisors Chairman John F. Herrity said yesterday he hopes to persuade private businesses in the county to foot the bill.