Abstract: A certain type of automobile shock absorber piston rod experienced abnormal fracture during bending test. The fracture cause was analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, metallographic examination, hardness testing. The results show that the piston rod absorbed hydrogen elements during the chrome plating process, but did not fully escape after hydrogen removal treatment. Hydrogen elements gathered at the inclusions in the high-frequency quenching zone, forming hydrogen induced cracks. Under the action of bending stress, the cracks continued to propagate, ultimately led to brittle fracture of the piston rod.