Abstract: Abnormal fracture occurred in the assembly process of an S45C steel automobile shock absorber piston rod. The fracture causes of the piston rod were analyzed by means of fracture analysis, metallographic examination, hardness test and chemical composition analysis. The results show that the microstructure of the matrix and hardened layer of the piston rod was normal, the thickness of the chromium plating layer and the hardened layer was within the allowable range, the chemical composition met the requirements of the relevant standards, and the material properties met the design specifications of the drawings. However, there were defects such as coarse cracks in the chromium plating layer on the surface of the piston rod. The main reason for the fracture of the piston rod was that the improper assembly operation led to the bending deformation of the piston rod, and the bending part bore a large tensile force. At the same time, the coating on the surface of the piston rod had defects such as pores, spalling, and coarse cracks, and was accompanied by hydrogen embrittlement cracks. Under the action of external tension, these defect sites became stress concentration points, which eventually led to sudden instability and fracture of the piston rod.