Abstract: To investigate the fracture cause of the inner 55CrSi springs of the lower body of a pump, the crack and fracture surface morphology of the broken springs were observed by stereomicroscopy and scanning electron microscopy. The results show that the fracture mode of the springs is fatigue fracture, and the cracks originated from the surface of the spring wires, which was caused by the dent and folding defects formed during the preparation and processing of the spring wires. In addition, the surface shot peening quality of the spring wires was poor, resulting in greater stress concentration. During the operation of the springs, the fatigue cracks propagated inward from the surface of the spring wires under alternating load, finally leading to the fracture.