An accurate nonlinear contact analysis model was established by using finite element method for a pair of spiral bevel gears of the tail reducer of a helicopter. Based on this model, dynamic meshing simulation was carried out in a meshing period. The performance of tooth contact fatigue and tooth root bending fatigue was studied. The regular pattern indicating the variety of tooth surface contact stress and tooth root bending stress with different meshing positions was presented. Then this paper analyzed the contact fatigue strength and the bending fatigue strength，and it was suggested that the major region of the fatigue damage was located in the side of tooth root compressed from the cloud figure of fatigue lifetime distribution. Finally, this paper obtained the node of tooth root bending fatigue life in the damage region before and after carburized machining.