Abstract: A small motor shaft of centrifugal fan broke after being connected online for 1 hour. The reasons for the fracture of the motor shaft were analyzed using macroscopic observation, scanning electron microscopy analysis, chemical composition analysis, mechanical property testing, metallographic examination, measurement of tool groove size and surface roughness. The results show that the fracture mode of the motor shaft was high cycle fatigue fracture, and the main reasons for the fatigue fracture of the motor shaft were off-axis rotation, rough machining of the tool groove, and low strength of the hot-rolled 45 steel.