Dress the tooth surface slightly along the tooth line direction to make it deviate from the theoretical tooth surface. The uneven distribution of load along the contact line of gear teeth can be improved by tooth modification, and the bearing capacity of gear can be improved. The methods of tooth profile modification mainly include tooth tip modification, spiral angle modification, drum modification and curved surface modification. Tooth tip thinning is the simplest modification method, but the modification effect is poor. Spiral angle modification is to slightly change the size of tooth direction or spiral angle β, so that the actual tooth surface position deviates from the theoretical tooth surface position. Spiral angle modification is better than tooth end modification, but because of the small change angle, it can not have significant effect everywhere in the tooth direction. Drum modification is to make gear teeth protrude in the center of tooth width through tooth direction modification, which is generally symmetrical on both sides. Although drum dressing can improve the uneven load distribution on the contact line of gear teeth, the dressing effect is not ideal because the load distribution at both ends of the teeth is not exactly the same and the error is not completely distributed according to the drum. Surface modification is tooth modification according to the actual eccentric load error. Considering the actual eccentric load error, especially thermal deformation, the trimmed tooth surface is not always convex, but usually concave and convex. The surface modification effect is good, which is an ideal modification method, but the calculation is more troublesome and the process is more complicated.