Abstract: A fracture fault occurred to a water pump shaft in a power plant. The fracture reasons of the water pump shaft were analyzed by methods of macroscopic and microscopic analysis of the fracture surface, chemical composition analysis, mechanical property testing, metallographic inspection and so on. The results show that:the microstructure defects, such as widmanstatten structure, reticulate ferrite and troostite distributed along the grain boundaries, existed in the fractured pump shaft, and the insufficient strength and toughness caused cracks in the sealing groove where the stress concentrated; under the action of alternating stress, fatigue cracking occurred to the shaft; the cavitation of the water pump accelerated fracture of the pump shaft during use.