Abstract: A main shaft of 40CrNiMo steel fractured during use. The fracture reasons of the main shaft of 40CrNiMo steel were analyzed by means of macroscopic observation, chemical composition analysis, hardness testing, and microstructure analysis. The results show that:the chamfer curvature radius of the main shaft was relatively small, and the nitrides existed in the material, which made the stress concentration more severe and caused the generation of the crack source; coarse grains decreased the cleavage fracture strength of the main shaft and increased the fatigue spreading rate, and finally the early fatigue fracture occurred to the main shaft. In the end, the improvement measures were put forward according to failure reasons of the main shaft.