Abstract: The shaft of a 30CrMo steel motor broke during operation. The causes of motor shaft fracture were analyzed using macroscopic observation, chemical composition analysis, mechanical property testing, low magnification inspection, metallographic examination. The results show that there were holes and crack defects on the surface of the motor shaft, uneven microstructure, caused poor mechanical properties of the material. Under continuous stress, microcracks were initiated at the defect site, and the cracks continued to propagate, ultimately leading to low stress high cycle fatigue fracture of the motor shaft.