The Fairfax County Park Authority board voted 5 to 3 last night to accept an agreement that would hand over much of its power to the County Board of Supervisors.

In response to the vote, Robert D. Moss resigned as the park board's chairman, complaining that board members had committed "a total about-face" from their previous insistence that the authority retain most of its power. Moss will stay on as a member of the board.

Calling the vote "a cave-in," Moss declared: "The agreement destroys the Park Authority . . . . It will lead to a tremendously politicized park system where the supervisors can use parks as a handout, a pork barrel . . . . "

The accord was approved after a 90-minute, closed-door session and was conditioned on the issuance by Virginia's attorney general of a favorable opinion on its legality. Such an opinion would have to be requested by the county supervisors. If the accord is finally approved, it will place Fairfax County Executive J. Hamilton Lambert in charge of the Park Authority. Lambert reports to the elected nine-member Board of Supervisors.

While the supervisors have portrayed the agreement as a management efficiency tool, Moss and some supporters of the Park Authority insisted it was intended to enable the county's politicians to exercise tighter control over the popular system of 342 parks on 14,000 acres.

The agreement would in effect transform the Park Authority into an advisory commission to prepare a budget, develop new facilities and programs, and recommend acquisitions.

Under the arrangement, which the supervisors are scheduled to vote on Monday, the Park Authority would be stripped of its power to appoint its officers and employes and to enter into contracts.

Park Authority board members who supported the agreement said it was better than the alternative -- the abolition of the Park Authority by the Board of Supervisors.

"If we go down the drain, then we have nothing," said Frederick M. Crabtree, an 18-year veteran of the park board, who voted for the agreement.

The Park Authority has had a large degree of independence. About half of its $15.3 million annual operating budget comes from fees paid by users of park facilities. The other half comes from the county's general fund.