Abstract: The planetary gear shaft of the final transmission system of a certain type of tractor fell off during use. The reasons for the chipping of the planetary gear shaft were analyzed by means of macroscopic observation, chemical composition analysis, mechanical property test, metallographic examination and scanning electron microscopic analysis. The results show that the reason for the chipping of the planetary gear shaft was that the heat treatment temperature of the part was excessively high, leading to the formation of an overheated structure, which reduced the impact toughness and led to the fracture of the gear shaft.