Basing on the reinforcement of a practical project, which is a reinforced concrete multi-span cantilever girder bridge, by changing section properties to improve the bearing capacity and seismic performance of the bridge. Then, a finite element model is established and solved by using the subspace iteration method, the frequency and vibration type is compared that of the bridge before and after reinforcement, the effectiveness of the new method on the structural dynamic characteristic is verified at the same time. Then, the seismic response of the bridge is calculated and analyzed by using the response spectrum method, which can provide method for reinforcement of similar bridge.