A rear wheel axle support assembly for a fitness bicycle includes: a front rack, a rear rack, and two retaining portions, on an upper portion of a left and a right rods of the front rack provided two coupling seats for connecting the left and the right rods of the rear rack, so that the front rack and the rear rack are connected in a herringbone pattern, the two retaining portions disposed at an upper end of the left and the right rods of the front rack for clamping both ends of a rear wheel axle of a fitness bicycle. The rear wheel axle support assembly is capable of clamping and unclamping the rear wheel axle of a fitness bicycle by using retaining portions.