Abstract: An 1Cr18Ni9 stainless steel plunger spring in a hydraulic pump fractured into 4 sections after service for 4 h.The fracture failure reason was analyzed by macroscopic analysis fracture observation,metallurgical structure inspection,hardness testing and measurement of the nitriding layer depth.The results show that the fracture mode of the spring was fatigue fracture,and the fracture origin was the axial direction crack of the spring inner side of surface.The cracking occurred during the process of drawing,and then stress corrosion which occured in operating environment promoted the propagation of the cracks,leading to the final fatigue fracture with the oil pressure circulating stress.