The Fairfax County Board of Supervisors voted unanimously yesterday to create a watchdog agency to oversee the county's planned 126-channel, $100 million cable television system.

The agency, called the Cable Television Administration Fund, will have seven employes, including an engineer, two management analysts and two inspectors and will be responsible for monitoring construction and analyzing finances of the cable company, Media General Inc., to which the supervisors awarded a franchise in September.

"They will be the overseers of the whole system, making sure we indeed do whatever we promised to do," said Lorraine Foulds, a spokeswoman for Media General.

Fairfax now has one man supervising a system expected to serve an estimated 125,000 subscribers when construction is completed in 1986.

The Administration Fund is supposed to be entirely financed by franchise fees from the cable company, but Media General will pay only $225,000 of next year's $294,000 budget. The balance will come from the county until Media General begins collecting revenues and pays the county back.

Alexandria and Arlington have two-member cable administration staffs that supervise the cable franchises there. Both system's are nearly complete, Alexandria's serving 15,000 subscribers and Arlington's about 23,500. Cable analysts say Fairfax's plans for a seven-member staff are reasonable for a county whose sophisticated cable system is expected to rival systems in cities like Pittsburgh and Milwaukee.

The board unanimously approved a 15-year lease that grants Media General a half-acre of county land rent free for a transmission facility in the 4600 block of West Ox Road. When the lease expires, the facility becomes property of the county.