Metro officials, gearing up to protect as much of their subway as they can from the Reagan administration's budget knife, said yesterday that $502 million that has already been spent will be thrown away if the system is cut from 101 to 62 miles as some have proposed.

Metro needs slightly more than $100 million in local, state and federal funds to bring a total of about 62 miles into operation. The additiona $502 million has been spent for work on various Metro lines that would carry the system beyond 62 miles, a Metro board committee was told yesterday.

The information was developed to make the case for Metro on Capitol Hill and before Transportation Secretary Drew Lewis, who has called Metro worthwhile but has promised no money.

The 62-mile system would include the 37 miles now in operation, plus:

The Red Line extension from Dupont Circle to Shady Grove in Montgomery County. That would open in two phases, from Dupont Circle to Van Ness Center in April or May of 1982 and from Van Ness Center to Shady Grove late in 1983.

The Blue Line extension from National Airport to Huntington in Fairfax County, scheduled to open late in 1982.

The Yellow Line shuttle, a second Potomac River crossing at the 14th Street Bridge that will connect the Pentagon and Gallery Place and open a north-south route under Seventh Street between L'Enfant Plaza and Gallery Place.

The following segments, part of the long-planned 101-mile system, would be eliminated if the system were cut to 62 miles.

The Orange Line from Ballston to Vienna, on which $133 million has been spent or obligated.

The Red Line from Silver Spring to Glenmont, on which $126 million has been spent or obligated.

The inner-city Green Line, from Anacostia through Southeast and Southwest Washington, north under Seventh Street to U Street, west under U to 14th Street, north under 14th to about Irving Street NW and then northeast to Georgia Avenue, on which $175 million has been spent or obligated.

The northern leg of the Green Line, from Georgia and New Hampshire avenues northeast into Prince George's County through Hyattsville and College Park to Greenbelt, on which $22 million has been spent or obligated. c

The southern leg of the Green Line, from the Anacostia station south to Rosecroft Raceway in Prince George's County, on which $6 million has been spent or obligated. A lawsuit also threatens that routing.

The Yellow Line from Alexandria's King Street station west to Van Dorn Street then south to Fairfax County's Springfield-Franconia station, near Springfield Mall, on which $40 million has been spent or obligated.

The Orange Line from Ballston to Vienna is the segment that is farthest along and easiest to finish. The least amount of work has been done on the Green Line, which serves the heart of the inner city.

Operating on the assumption that more than 62 miles will be built, John S. Egbert, Metro's assistant general manager for design and construction, outlined two four-year construction programs: The one he would like and the one that most Metro officials think they will probably get.

The one Egbert would like would cost the federal government $350 million in fiscal 1982, Reagan's first budget, and a bit more in each succeeding year. i

At the end of four years, funds would have been provided for the 62 miles plus the Orange line to Vienna (late in 1986); the Green Line from Anacostia to Georgia Avenue (late in 1987), and all of the Red Line tunnel work from Silver Spring to Glenmont but only the Forest Glen station (early in 1987).

The program Metro officials think they will get provides for $275 million in federal dollars in fiscal 1982, plus a little more in each succeeding year. At the end of four years, the 62 miles would be in operation, the Orange Line to Vienna would open late in 1986, structural work, but no stations would be completed on the Green Line from Anacostia to Georgia Avenue, and the Red Line from Silver Spring would be completed only as far as Forest Glen and open early in 1987.

Neither program completes the Greenbelt or Rosecroft Raceway legs of the Green Line or the Yellow Line from King Street to Springfield-franconia. g

The Metro budget committee also adopted a bus and subway operating budget of $318.9 million for the year beginning July 1. That is a 15.5 percent increase over this year. The budget anticipates a fare increase in January 1982 and a subsidy from local, state and federal sources totaling $161.9 million. Riders will pay 50.7 percent of the budget in fares.

The committee cut 105 new jobs Metro general manager Richard S. Page had proposed. All the jobs, Page said, were needed for scheduled expansion of the subeay. He promised more study on the matter, but warned that the cuts could mean a delay in the opening of the Red Line extension to Van Ness Center.