Abstract: A fracture accident occurred in the first-stage piston rod of a hydrocracking new hydrogen compressor in a petrochemical enterprise. The fracture causes of piston rod were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the fracture property of the piston rod belonged to fatigue fracture. The main reason for the fracture was that the primary cylinder or the middle body had sunk, resulting in the coaxial deviation between the cylinder and the middle body. The secondary reason was that the strength and impact properties of the piston rod material were low, and the hardness was high, which shortened the fatigue life of the piston rod.