The Reagan administration's mass transit chief urged Metro officials yesterday to revise their long-debated plans for subway construction and complete the central section of the Green Line between Anacostia and Northwest Washington by tapping funds previously appropriated by Congress.

"There is adequate funding to complete the Green Line from U Street to Anacostia," said Ralph L. Stanley, who heads the Urban Mass Transportation Administration. "They should have built that first."

Delays in constructing the Green Line, the Metro system's only unopened route, have long caused concern among federal and local officials because the line is designed to serve such low-income areas as Shaw and Anacostia, where many residents depend heavily on public transportation.

Stanley's statement marked the first federal proposal for revamping the transit agency's controversial plans for expanding the rail system since the Reagan administration recommended halting federal spending for Metro construction.

In December, federal officials proposed cutting off federal funds for Metro after the current fiscal year, as part of an effort to comply with the Gramm-Rudman-Hollings deficit reduction measure.

President Reagan's budget, sent to Congress yesterday, would provide no further appropriations for Metro.

The administration's moves have sparked widespread protests from Metro and other local officials and from Maryland and Virginia members of Congress.

The proposed cutoff of funds has threatened to derail Metro's plans to build subway extensions in Prince George's County and Northern Virginia.

"We are very carefully analyzing all of our options," said Metro General Manager Carmen E. Turner, adding that the authority will ask Congress to overturn the administration's proposal.

Turner said the transit agency still plans to press for funds to complete the proposed 103-mile rail system.

At a news conference, Sen. John W. Warner (R-Va.) denounced the proposed halt in spending, saying, "We're going to fight it." Rep. Michael D. Barnes (D-Md.) termed the move a "shortsighted proposal" that would "rob our region." And Prince George's County Executive Parris Glendening threatened a court challenge.

Stanley's proposal was triggered partly by a letter to Reagan last month from Metro officials. The letter warned that the proposed funding cutoff would jeopardize the Green Line.

Stanley's recommendation appeared designed to shift to Metro the onus for any failure to open the Green Line.

In a letter released yesterday, Stanley said that $541 million in federal and local funds are available for Metro construction, far more than the $285 million needed to complete the Green Line between Anacostia and a U Street station planned between 10th and 13th streets NW.

Federal officials said that sufficient funds also are available to complete a Red Line extension to Wheaton in Montgomery County. However, Metro would not have enough federal funds to carry out other parts of its plan to expand the system to 89.5 miles by the early 1990s, they said.

As a result, these officials said, Metro may have to halt construction of a Green Line extension from Fort Totten in Northeast Washington to Greenbelt in Prince George's. Metro also may need to set aside its plans for a Yellow Line spur to Alexandria's West End, they said.

Stanley has refused to release nearly $400 million in funds previously appropriated by Congress for Metro. He said that he will release the money if Metro revises its plans or obtains financial guarantees from the District, Maryland and Virginia to provide local funding if Metro's federal allotments run out.