Abstract: When an automobile used for two days and traveled 56 km, it was found that the shaft of the right rear trailing arm fractured. The fracture reasons of the shaft of the right rear trailing arm were analyzed by macro and micro fracture examination, hardness testing, metallographic examination, chemical composition analysis and so on. The results show that:the main reasons for the fracture of the shaft were that the defects existed in the forging structure and the quenching and tempering heat treatment didn't carry out, which greatly reduced the strength of the shaft; the secondary reason was that the surface grinding hardening layer formed on the surface of the shaft due to the overheating grinding, which increased the brittleness and tensile stress of the shaft surface, and made the micro cracks appear on the shaft surface.