Abstract: Analyze the chemical composition, mechanical properties, macroscopic morphology of the fracture,scanning data of the fracture and usage conditions of the failed motor spindle. The results show that the fracture exhibits a fatigue fracture morphology. There are obvious crack sources on the fracture, and obvious fatigue striations in the propagation area. There are X-shaped cracks on the surface of the motor spindle, and the crack propagation direction forms an angle of 45° with the axial direction of the motor spindle. The fracture has the characteristics of torsional fracture. The cause of failure is that the motor spindle generates torsional vibration fatigue cracks under the influence of resonance.