Abstract: The fracture phenomenon of the main gear shaft of a certain type of product was analyzed by means of macroscopic observation, scanning electron microscopy and energy spectrum analysis, metallographic examination, mechanical property test and chemical composition analysis. The results show that the main reason for the fracture of the main gear shaft was abrasive wear in the process of tooth surface transmission, which causesd the load to change abruptly, and then formed a fatigue source at the stress concentration of the tooth root. Under the repeated action of alternating stress, the crack initiated and expanded from the fatigue source, and with the change of the force direction during the expansion process, the direction of the fatigue crack also changed continuously. The stress of the remaining section of the tooth exceeded its ultimate stress, which eventually led to the fracture of the gear.