Research conclusions:(1) The horizontal crack on the web plate of longitudinal beam is caused by damage by hard matter and the bottom vertical crack on the web plate in end section of longitudinal beam is caused by fatigue. (2) The bedded crack on low plain longitudinal node plate is caused by rejected steel quality and slag inclusion in interlayer while smelting. (3) The etched concave surface on cross beam is caused by long-time erosion of sewage on the bridge dropping from train.(4 ) The internal force distribution of longitudinal beam is changed and stress level of longitudinal beam is improved by adopting the method of pasting steel plate on crack position for reinforcing longitudinal beam. In this way, the bridge can meet the requirement of bearing capacity.