The Senate, in its budget-cutting plan adopted early yesterday, voted to reduce federal spending for Metro subway construction by $62.5 million next year, jeopardizing the transit authority's long-debated proposal for expanding the rail system to 89.5 miles.

The Senate measure, which drew objections from some Metro advocates in Congress, would cut outlays for subway construction by 25 percent to $187.5 million in fiscal 1986. Metro's 89.5-mile plan was designed to be financed at an annual level of $250 million in federal funds.

A Metro spokesman warned yesterday that a major cutback by Congress would lead to delays and higher costs for rail construction. The 89.5-mile plan includes a proposed Green Line extension to Greenbelt in Prince George's County and a Yellow Line spur to Alexandria's West End.

The Senate also approved a 20 percent cut in federal operating subsidies for Metro and other transit systems throughout the United States, along with a 27 percent reduction in other mass transit grants. Metro officials previously said these moves might result in higher fares or a curtailment in service.

Under existing formulas, Metro is allotted $18.5 million a year in federal operating aid. The authority relies on more than $50 million in federal grants annually to buy buses, purchase rail equipment and build bus garages.

Sen. Charles McC. Mathias Jr. (R-Md.), who heads a subcommittee that oversees Metro authorizations, voted against the deficit-cutting plan, and a Mathias spokesman cited the Metro cuts as "an example of why" he opposed it.

The measure is expected to face opposition in the House. An aide to Rep. Steny H. Hoyer (D-Md.), who has sought federal funds to expand the subway system, said Hoyer is "relatively confident" that the House will block any reduction in Metro construction outlays.

The Senate cut, previously endorsed by the Reagan administration, marked a turnabout in prospects for Metro's 89.5-mile plan. In March, the administration had agreed to negotiate a contract aimed at financing the planned extensions. Metro officials now are drafting contract proposals.

At the same time, Metro officials have begun studying possible issuance of bonds to help finance rail construction. If such a move is authorized, it could offset cuts in federal appropriations, officials have said.

The Senate's 20 percent cut in operating subsidies was less severe than an earlier administration-backed proposal, which called for a five-year phase-out. Nevertheless, an American Public Transit Association spokesman described the cutbacks as "devastating."