The invention discloses a reinforcing structure for a corbel end of a cantilever girder bridge and a mounting method for the reinforcing structure and relates to the technical field of bridge reinforcement. Each pair of rigid beams of one group of reinforcing structure are arranged between two adjacent permanent supports along the extension direction of the bridge; a beam support is arranged between each rigid beam and a bridge pier; a beam support is arranged between two ends of each rigid beam and a concrete beam respectively; all rigid beams support the corbel adjacent to the bridge pier. The mounting method for the reinforcing structure comprises the following steps of arranging a lifting platform and lifting the rigid beams to designed positions; shifting the rigid beams to preset positions on the top surface of the bridge pier; arranging the beam supports between the rigid beams and the bridge pier and between the rigid beams and the concrete beam. The corbel end is actively reinforced to avoid damage to the existing bridge, and the usage of the bridge is not influenced in the construction process.