The Fairfax County Park Authority -- considered by some park officials to be one of the best in the nation -- could lose its semi-independent status under proposals now being studied by the county Board of Supervisors.

Conservationists and some citizen groups have called the move "a power grab" by elected officials who want to exert more political control over park decisions and may want to reduce county support for parks and recreation.

Supervisor's Chairman John F. Herrity, who heads a three-member sub-comittee studying the possible take-over of the park authority, insisted this week that those fears were unfounded.

"I'm a strong supporter of parks and I think basically the park authority is doing a good job," he said. "We're just trying to tighten things up, end duplication and get the biggest bang for the buck."

Last week, Herrity told a meeting of park authority officials that most supervisors favor at least a partial takeover of the authority.

So far there hasn't been any specific criticism of the park authority or its operations, and the issue has arisen only because the original 30-year charter of the park authority is up for recertification on Dec. 6.

"It's a philosophical and political issue . . . they want to get rid of independent kingdoms and fiefdoms," said one county official.

The board recently ended the semi-independent status of mental health and social service boards and is expected to do the same to the semi-autonomous library board. Like the park authority, these boards are chartered as semi-independent agencies under state laws, but their budgets and members are approved by the Board of Supervisors.

"The thing that bothers me is that when you have what is considered to be one of the finest park systems in the country, with the quality of its programs and professional staff the envy of the nation, why do you want to change it?" asks John Davis, executive director of the National Recreation and Parks Association, which has its headquarters in Arlington.

"When in hell would the Board of Supervisors have the time to give the attention to the parks that the park authority board is giving and the parks deserve?"

Eliminating the semi-independent park authority also could affect donations. More than 400 acres of parkland, worth more than $6 million, has been given to the park authority in recent years and put in a trust fund for park use.

"I can't see citizens or developers giving land to the county when there's no guarantee of it becoming a park," said former supervisor Jean Packard, a board member of the Federation of Citizen Associations.

Packard also was concerned tghat funding of the park system would decrease. The authority presently, under its state charter, must put park revenues in a trust fund that can be used solely for park purposes. As a county department those revenues -- $3.6 million in fiscal 1980 -- would go into the county's general fund.

The park authority's current director, Joseph Downs, and its present board chairman, Estelle Holley, both say they don't mind the supervisors' review of the park authority and insist that if any duplication or problems are found they would act immediately to resolve them.

But both also feel the present system is working well.

The park system's operating budget this fiscal year is projected to be more than $8 million, almost half of it supported by fees charged at 18 to 20 of the major parks.