A Senate Appropriations subcommittee voted yesterday to cut federal funding for Metro construction over the next year to $185 million, which would be used to help complete the long-awaited Green Line to Greenbelt.

The action by the transportation subcommittee would represent an 8 percent reduction from the $201 million allotted to the transit agency in fiscal 1987, which ended yesterday.

The subcommittee's action now goes to the full committee, which is expected to endorse the recommendation and send it to the floor, where the full Senate also is expected to approve it. The House has voted to appropriate $197 million for Metro construction in fiscal 1988, so the two measures would have to be reconciled in conference.

Sen. Barbara A. Mikulski (D-Md.) said last night: "I am optimistic that we can bring the Metro funding figure up to the House level in conference."

In the past, the two chambers have split the difference in their bills. If they follow suit this time, it would produce a compromise of about $191 million, or 5 percent less than Metro received last year.

"We hope in conference that the bill comes out as high as possible," said Metro spokeswoman Marilyn Dicus. "We have a favorable bidding climate and want to build as much of the system as fast as we can."

The Senate subcommittee's $185 million figure represents "a substantial part of what we asked for," Dicus said.

Metro had asked Congress for $250 million in fiscal 1988 to continue expanding the system, currently 69.6 miles, to 89.5 miles by the early 1990s. Metro officials plan to start lobbying Congress soon for an additional $2.2 billion in federal funds, as part of a $2.7 billion plan to extend the system to 103 miles by the year 2002.

The Reagan administration, which failed in an earlier attempt to halt federal funding of Metro construction, had urged Congress to limit Metro to $130 million in fiscal 1988.

Metro plans to use the money to build the three-mile segment of the Green Line from College Park to Greenbelt in Prince George's County. Metro hopes to open the line by late 1993.