The invention discloses an on-orbit frequency increasing mechanism for a spacecraft flexible appendage. The on-orbit frequency increasing mechanism comprises a driving element, a speed reducing device, a support frame, a woven stay cord belt, a joint support, a transitional shaft section, a unidirectional bearing and a winding reel, wherein the driving element, the speed reducing device, the joint support, the transitional shaft section, the unidirectional bearing and the winding reel are respectively mounted on the support frame, the woven stay cord belt is wound on the winding reel, driving power of the driving element is transmitted to the winding reel via the speed reducing device, the transitional shaft section and the unidirectional bearing, the winding reel rotates to drive the woven stay cord belt to be tensioned and loosened, and an end portion of the woven stay cord belt is connected with the flexible appendage. By the on-orbit frequency increasing mechanism for the spacecraft flexible appendage, the problem that the spacecraft flexible appendage is too low in fundamental frequency or frequency is required to be improved for coupling of lower-order frequency and on-satellite disturbance source. Meanwhile, the on-orbit frequency increasing mechanism for the spacecraft flexible appendage has the advantages of light weight, simplicity in control, small size, capability of being unfolded along movement, low consumption and the like, and can effectively guarantee reliability and applicability of the spacecraft flexible appendage in the on-orbit frequency increasing mechanism.