To find out the cause of pump shaft failure, this paper analyzed the failure of the pump shaft with SEM, metallographic microscope, direct reading spectrometer, micro hardness tester and other test methods. The structure, chemical composition, and micro-hardness were analyzed. The results showed that the fracture morphology had obvious brittle fatigue cracking characteristics, and the crack source had multi-source characteristics, fatigue fringes and secondary cracks existed. The material composition and microstructure of 316L pump shaft were not significant, and there were a lot of inclusions on the near surface. At the same time, pitting and microcracks were observed on the surface of the pump shaft. Near-surface inclusions are prone to pitting corrosion in an acidic environment, and there is a variable cross-section at the junction of the pump shaft and the coupling root to form stress concentration. When the pump shaft is subject to corrosion, stress and alternating load of the motor, a source of corrosion fatigue crack is formed. The instantaneous fracture caused by crack propagation is the main cause of the fracture of the stainless steel pump shaft.