Fairfax County officials yesterday lifted all restrictions on outdoor water use in the county following the weekend's rain which replenished Northern Virginia's main reservoir.

Alexandria and Prince William County, which imposed smiliar controls last month, are expected to take similar action today.

The more than 700,000 persons served by the Fairfax County Water Authority have been prohibited from watering lawns, washing cars and other outdoor uses of water since Jan. 28, when the agency's drought-stricken Occoquan Reservoir fell to less than a third of its 11-billion gallon capacity.

But the weekend of rain, which continued through yesterday, plus an inch of prescipitation two weeks ago, has replenished the reservoir. Water authority spokesman James Arfield said runoff from the current rain should fill the reservoir by Wednesday.

The wet weather of the past two weeks should total about four inches of rain, or four times as much as fell altogether during the unusually dry months of December and January. The heavy pattern of rain bore out a long-range Service late last month which said that February should produce above normal precipitation in the area.

While Warfield said the water authority should have no further supply problems during the rest of winter or spring, he said the agency could have trouble meeting peak demand this summer.

He explained that even if there is enough water in the reservoir, the authority cannot meet unlimited demand because it has to treat that water and pump it through the pipeline network. "We couldn't meet peak demand last Labor Day," Warfield said. "That could happen again."

To meet such problems, the water authority is constructing a new treatment plant near the Potomac River. The Plant, which would use water pumped from the Potomac instead of the Occoquan, will not be operational until the summer of 1982.