The invention discloses a reinforcing structure for improving bearing capacity of a hollow slab girder bridge and a construction method thereof, and relates to the technical field of bridge maintenance and reinforcement. The main purpose of the reinforcing structure is to improve the bearing capacity of existing bridges without causing economic waste and without disrupting traffic. According to the main technical scheme, the reinforcing structure comprises a beam body, a plurality of reinforcing plates, supporting portions and a plurality of reinforcing beams, wherein the beam body is composedof a plurality of hollow plates which are connected side by side with one another, and a hinge joint is formed between every two adjacent hollow plates; the reinforcing plates are connected side by side to the bottom face of the beam body, are respectively parallel to the hinge joints and cover the hinge joints; the supporting portions are arranged under the beam body, and are connected with cover beams of the bridge; and each reinforcing beam is I-shaped structural steel, the upper flange of each reinforcing beam is connected with the bottom face of each hollow plate, the lower flanges of the reinforcing beams are supported on the supporting portions and the reinforcing beams are parallel to the hinge joints. The reinforcing structure is mainly used for improving the bearing capacity ofthe existing bridges.