Taking the seismic design method of small and medium span beam Bridges as the research object, a systematic and comprehensive earthquake damage survey of small and medium span beam Bridges in Wenchuan area is carried out, the earthquake damage forms of beam Bridges are summarized, and the damage conditions of various components are analyzed by statistical analysis. It is proved that the beam Bridges in Wenchuan earthquake have more main beam displacement, more earthquake damage of supports and stoppers, less earthquake damage of piers, and main beam fall Less prominent features; By means of shaking table test and numerical simulation, the mechanism of earthquake damage of beam bridge is studied. The results show that the relative sliding between the main beam and the plate rubber bearing plays a seismic isolating role, reduces the seismic internal force response of the pier, and significantly reduces the earthquake damage of the pier. The bridge floor continuously coordinates the displacement of the main beam, and the abutment constrains the longitudinal displacement of the main beam, which is the main reason for the few falling beams in the Wenchuan earthquake. Based on the seismic damage analysis and mechanism research of small and medium span beam bridge, the seismic design concept of using the limited relative sliding between the main beam and the support to reduce the acceleration response of the superstructure and reduce the pier damage is proposed.