Abstract: Aiming at the tooth fracture accident of the middle gear shaft in the marine gearbox, the causes of the tooth fracture were analyzed by means of macroscopic observation, chemical composition analysis, scanning electron microscopy and energy spectrum analysis, hardness test and metallographic examination. The results show that there was Al₂O₃ slag on the subsurface of the middle gear shaft, and the alternating stress existed in the gear during operation, which caused the microcracks to initiate in the gear, and the microcracks further fatigue expanded, eventually leading to the fracture of the gear.