The Metro system plans to begin rail service on the Yellow Line to Alexandria and Fairfax County Dec. 17 under new proposals adopted yesterday by a special committee of the transit authority's board of directors.

The transit authority's plans for opening the four-mile extension between National Airport and Fairfax County's Huntington station had recently been thrown into doubt because of delays in testing new rail cars. Metro officials concluded, however, that they could start service on time by using shorter trains than previously proposed, which would require fewer new cars.

Transit officials cautioned yesterday that the opening of the Huntington spur may initially lead to overcrowding during rush hours on portions of the Blue Line. This would occur, they said, if a large number of passengers who board trains at the newly opened stations transferred from the Yellow to the Blue Line at the Pentagon stop.

If subways become crowded, officials said, the transit authority will move quickly to lengthen the trains to accommodate more passengers on the Blue and Yellow Lines as additional rail cars become available.

The plans for the Dec. 17 opening, drawn up by senior Metro officials, are expected to be formally accepted by the board next week.

The board's special committee also endorsed a proposal aimed at ending confusion among subway riders about the Blue and Orange Lines' intertwining routes.

At present, trains are designated as operating on the Blue Line when they run in one direction, and are designated Orange Line trains when they run in the opposite direction. Under the new proposal, no switch from Blue to Orange would occur.

Blue Line trains would operate in both directions between National Airport and Addison Road. Orange Line trains would run in both directions between Ballston and New Carrollton. This move is also scheduled to be effective Dec. 17.

The plans for the Huntington extension's opening have caused concern among government officials and commuters partly because service has already been delayed by a year. The extension, which includes three stops in Alexandria, was scheduled to open in December 1982, but it was postponed because of delays in deliveries of new rail cars. The Alexandria stations are at Braddock Road, King Street and Eisenhower Avenue.

The Metro system recently planned to open the Huntington spur by operating six-car trains, a proposal that would have required 36 new rail cars. Under the new plan, the transit system will initially operate four-car trains to Huntington and, officials said, only 10 new cars will be needed.