Abstract: The impeller shaft of a water pump produced by a company broke after more than half a year of service. The causes of fracture were analyzed by means of macroscopic observation, chemical composition analysis, macroscopic examination, scanning electron microscope analysis, metallographic examination and hardness test. The results show that the quenching and tempering treatment process of the impeller shaft in the manufacturing process did not meet the requirements, so that a large number of bainite structures with high brittleness and low hardness were formed on the surface of the impeller shaft. In addition, the fracture source was located at the retreating groove of the impeller shaft process, and the retreating groove had turned all the hardened layer, which further reduced the surface strength of the shaft and was easy to form stress concentration at the retreating groove. The impeller shaft was subjected to a large torsional force during operation, and fatigue cracks were easy to form on its surface. With the crack propagation, the pump impeller shaft finally broke.