The Reagan administration has agreed to reduce spending for Metro by more than $50 million next year, throwing into doubt the transit authority's plan for expanding the subway system to 89.5 miles.

Ralph L. Stanley, the administration's mass transit chief, told a Senate subcommittee yesterday that federal funds for Metro construction would be cut by 25 percent as a result of an agreement earlier this month between the White House and Senate Republicans aimed at curbing deficits.

Stanley's statement, at a hearing by the Appropriations subcommittee on transportation, was the first indication that the administration viewed the budget compromise as including a major cutback in Metro spending.

The administration previously had recommended allocating $250 million annually for Metro over the next four years -- a level in line with the transit agency's 89.5-mile plan. The funds are needed to build a Green Line extension to Greenbelt in Prince George's County and a Yellow Line spur to Alexandria's West End.

But Stanley told the subcommittee that Metro's allotment would be cut to $196.5 million under the Senate-White House agreement. A spokesman later said that the amount might be further reduced to $187.5 million. The difference between the two figures hinges on possible adjustments for inflation, the spokesman said.

The proposed cutback appeared to represent a shift from the administration's agreement last month to negotiate a contract with Metro aimed at financing the 89.5-mile plan. Metro officials recently began drawing up a contract proposal as an initial step in the complex negotiations.

The 89.5-mile plan is based on the current $250-million spending level, an amount already below the authority's requests. It is unclear whether 89.5 miles can be completed as planned if federal appropriations are significantly reduced, officials said.

A spokesman for the Urban Mass Transportation Administration declined to elaborate on Stanley's testimony. Stanley has repeatedly urged Washington area officials to provide more county, city and state funds to finance Metro construction.

Metro General Manager Carmen E. Turner expressed concern over Stanley's statement, warning that cutbacks would lead to delays and higher costs for subway construction. "We will continue to make the case for the $250 million," she said.

An aide to Rep. Steny H. Hoyer (D-Md.), who has pressed for federal funds to expand the system, said Hoyer will seek to block any cut in Metro's appropriation. "We view that as a flagrant break of commitment by the administration," the aide said.

The deficit-cutting compromise, scheduled for Senate debate next week, is expected to face stiff opposition.

Officials said changes in controversial mass transit proposals are possible. In addition, the compromise sets overall goals, leaving specific appropriations, including that for Metro, open to review.

Stanley told the subcommittee that the proposed shifts in spending for Metro and other transit systems were "acceptable only as part of the total budget compromise package. If significant changes are made elsewhere in the package, the transit portion would be jeopardized."

Stanley said the compromise also included a five-year phase-out of federal subsidies for mass transit operations and cuts in other grants from nearly $2.7 billion this year to $1.7 billion in fiscal 1988. CAPTION: Picture, METRO'S CARMEN E. TURNER . . . still seeks promised $250 million