Ralph L. Stanley, the Reagan administration's mass transit chief, said yesterday that he believes Congress will provide additional federal funds next year to expand the Metro subway system, despite opposition from the White House.

President Reagan has recommended cutting off all federal spending for Metro construction after the current fiscal year. The president's proposal has threatened to derail plans drawn up by the Washington Metropolitan Area Transit Authority (WMATA) to expand the rail system to 89.5 miles by the early 1990s.

But at a congressional hearing yesterday, Stanley, who heads the Urban Mass Transportation Administration, conceded that Congress is likely to balk at the administration's plan.

"Will there be funds appropriated for WMATA? I would say, 'Yes,' " Stanley said in response to a question by Rep. Michael D. Barnes (D-Md.), a Metro advocate. Stanley described his comment as a personal opinion and said it did not reflect any shift in the administration's opposition to Metro spending.

In an interview, Stanley cited widespread congressional opposition to the president's fiscal 1987 budget and added that the administration would face "an uphill battle to get zero" funds for Metro.

The Senate Budget Committee, in an initial move, voted last week to reject the administration's mass transit proposals and allocate $217.2 million next year for Metro construction, the same level as Metro's current appropriation. The House and Senate appropriations committees have not considered the issue.

Barnes, who presided at yesterday's hearing by a House District subcommittee, and Rep. Steny H. Hoyer (D-Md.) both cited the Senate move as evidence that the administration's recommendation will be overturned. "Nobody in Congress believes we're going to zero fund Metro," Barnes said.

At stake are Metro's plans to complete two sections of the Green Line in the city and Prince George's County, along with a Red Line extension to Wheaton and a Yellow Line spur to a Van Dorn Street station in Alexandria's West End.

One of the Green Line sections would connect Anacostia with a U Street station in Northwest. The other segment would link Fort Totten in Northeast with a Greenbelt terminus in Prince George's.

The administration's move to cut off Metro spending drew protests yesterday from D.C. Mayor Marion Barry, Montgomery County Executive Charles W. Gilchrist and Rep. Frank R. Wolf (R-Va.).

However, Rep. Stewart B. McKinney (R-Conn.) objected to Metro's mounting construction costs, saying, "Congress won't pay for it."

In other developments:

* Barnes and McKinney expressed concern about a move by Metro officials to spend millions of dollars for new Farecard equipment without competitive bidding.

At a closed session last week, the Metro board tentatively approved a plan to buy the fare collection devices from Cubic Corp., the manufacturer of the existing Farecard machines, sources said. The proposal has triggered objections from Alta Technology Inc., which wants to compete for the contract.

* Stanley charged that Metro had revised its long-term plan for completing the system to include 104 miles at a cost of $12.5 billion -- a contention disputed by Metro officials. The proposed system will extend 103 miles at an estimated cost of $9.4 million, Metro officials said.

The dispute hinges on whether interest payments on bonds issued to finance subway construction are included, federal and Metro officials said.