Abstract: The pump shaft of 316L stainless steel was fractured after running for three months. By using metallographic microscope, scanning electron microscope, X-ray fluorescence spectrometer and other detection means, the fracture reason of the shaft was analyzed. The results showed that the main reason of fatigue brittle fracture of pump shaft was that austenitic matrix had a lot of distribution along the grain boundary of the second brittle phase of δ.