Prince George's County, fearful that future federal cutbacks could mean that most of its share of Metrorail track never will be built, is holding up an area-wide construction agreement to press for accelerated purchase of land and building of stations in the county.

The county's apparent goal is to assure that construction on the track it feels is threatened -- the Green/Yellow Line to Greenbelt and the Green Line to Rosecroft -- is far enough along by the mid-1980s that the federal government would have little choice but to finance its completion.

At a closed-door meeting of the system's eight jurisdictions yesterday, county representatives proposed diverting close to $100 million over three years to the two sections. Much of that sum would come from Montgomery County's Red Line extension to Glenmont, slowing completion there by a year.

Montgomery County Executive Charles Gilchrist declined comment on the proposal until he has had time to study it.

Federal funds currently cover most of Metro's mammoth construction costs. Ever since the Reagan administration announced earlier this year that it would commit itself, for the time being, only to a 75-mile system (Metro's design calls for 101 miles), Prince George's officials have expressed serious concern.

If the system does stop at 75 miles, the Rosecroft and Greenbelt extensions, as the last in the system to be built, would be among the most likely candidates for elimination. Their track in the county would run about nine miles. Blue and Orange Line track currently in operation in Prince George's County is about five miles long.

The administration has not defined precisely what it thinks should make up a 75-mile system. In addition to the Prince George's County mileage, however, the most vulnerable sections would be Green Line segments in the District of Columbia and the Yellow Line from Alexandria to Franconia-Springfield.

Final plans for the southern Green Line have been delayed for years by lawsuits and political wrangles over its precise route. As a result, Prince George's had given its approval to construction in other jurisdictions on the understanding that in the end it would get its full share of the system.

Metro staff members said they had thought the system's eight jurisdictions were near agreement in principle on a $1.3 billion, three-year construction schedule. But at yesterday's 8:30 a.m. meeting of the Ad Hoc Committee on the Capital Construction Program at Metro headquarters, Prince George's County member David Hartlove introduced the county's new stand.

"Our attitude," he said yesterday after the meeting, "is if you're not going to get any more, why keep paying?. . . We will gain nothing unless we demand it." Ray La Placa, another county representative at Metro, said Prince George's had paid about 55 percent of its share of construction costs but so far had only about 31 percent of its tracks completed or committed.

The committee normally sends recommendations to the full Metro board by consensus, not a formal vote. Prince George's representatives, therefore, would appear to have the power to hold up agreement indefinitely. Ultimately, County Executive Larry Hogan could withhold his signature from federal applications, which effectively would block the system's major source of capital funds.

Metro General Manager Richard S. Page said that current thinking in Metro foresees a construction spending of $370 million, $460 million and $470 million in each of the coming fiscal years, though federal funds for only the first year appear assured.

That would complete the system to 63 miles -- the Blue Line would extend to Huntington, the Red Line to Shady Grove and the Orange Line to Vienna, and the Yellow Line shuttle would begin service, crossing the 14th Street Bridge to link Gallery Place and National Airport.

In addition, substantial money would go to construction on the Red Line through to Glenmont, the Green line between Anacostia and U Street and the Yellow line to Franconia-Springfield, routes that are scheduled for opening sometime in the late 1980s.

According to a committee source, proposals put forward by Prince George's yesterday would move up purchase of land for the Greenbelt and Rosecroft extensions to the first and second years of the three-year building program, rather than the second and third year.

It would also commit funds for construction of stations at West Hyattsville and Prince George's Plaza to the plan's second and third year. Previous proposals had that construction delayed until the plan's third year and the year after that.

In fiscal year 1982, Prince George's wants $37 million for land purchases, against the $8.1 million now proposed, according to Hartlove. In fiscal '83 it is seeking $47 million construction funds as opposed to $12.6 million. For fiscal '84, it would want another $47 million for construction, according to Hartlove.

If Prince George's proposals are adopted, the three-year plan would cover purchase of all land needed for the full 101-mile system, design of most of the stations and options on additional rail cars. This, county officials hope, would be additional incentive to the federal government to finish the system in its entirety.

The new proposals would mean slowing development in other jurisdictions, primarily Montgomery County. The Red Line extension to Glenmont, now scheduled to as far as Wheaton in 1988, would be delayed by one year. Funds for acquisition of land for the Franconia-Springfield extension of the Yellow Line also would be reduced.

The Metro board is scheduled to consider the capital program on Aug. 13.