The impact and overload of the gear make the maximum shear stress increase and the peak stress moves to the deep. The surface structure is coarse, the hardened layer is shallow, and the hardness gradient is steep. The above factors cause the local stress of the transition layer to exceed the gear strength, and cracks form and expand at the weak points, resulting in spalling. The solution is to prolong the diffusion time of gear carburizing; to extend the tempering time to 4-6h, to ensure that the quenched martensite is fully tempered and improve the toughness; to ensure that the gear is lubricated enough and contact well during operation, so as to reduce the occurrence of impact and overload.