Abstract: During the operation of a certain type of fan, the gear shaft in the gearbox had broken teeth. The causes of tooth breakage were analyzed by macroscopic observation, fracture analysis, energy spectrum analysis, hardness test and metallographic examination. The results show that the fracture characteristics of the gear shaft were fatigue fracture. There were large-scale B-type alumina inclusions at the crack source, which caused obvious stress concentration, and led to the broken tooth of the gear shaft. Finally, the rectification measures were put forward. After rectification, there was no broken tooth in the fan gear shaft.