Abstract: Macroscopic and microscopic morphology analysis, material performance test, metallographic examination, bolt load checking and other methods were used to analyze the fracture reason of the pressure cover bolt of a dilute pump. The results show that the high cycle fatigue leaded to the fracture of the bolt of the pressure cover of the dilute pump. The thickness of the decarburized layer on the surface of the bolt thread exceeded the standard and decline of material properties promoted fatigue fracture of bolt.