Abstract: The shaft of a 42CrMo fan produced in 2014 broke during operation, which directly affected the operation of the fan. Combined with the working conditions of the spindle, the macro and micro morphology of the spindle fracture was observed, and the chemical composition analysis, microstructure observation, hardness and tensile properties test of the spindle material were carried out. The results show that the mechanical properties of the shaft are uneven due to the inhomogeneous distribution of the shaft structure. At the same time, there are obvious turning grooves in the arc transition of the shaft shoulder, which form serious stress concentration when the spindle is running, resulting in the fatigue fracture.