Abstract: Aiming at the problem of frequent fracture of water ring vacuum pump shaft, the fracture causes of the pump shaft were analyzed by macro observation, metallographic examination, scanning electron microscope and energy spectrum analysis. The results show that the main reason for the fracture of pump shaft was that in the environment containing elemental media such as chlorine and nitrogen, there was local enrichment of Cl^- in the gap near the thread undercut of pump shaft, which accelerated the corrosion. At the same time, under the combined action of stress concentration and fatigue alternating load, cracks on pump shaft were produced and extended to fracture.