Aiming at the problem of large bending stress at the tooth root of transmission gear, the helical gear model of a commercial vehicle transmission is established by 3D modeling software SOLIDWORKS and imported into finite element analysis software ANSYS, and the fatigue life of gears made of four kinds of steel materials is explored respectively. Comparing the stress value obtained by finite element analysis with the theoretical stress value, the stress value of ANSYS static analysis is 204.77 MPa, the theoretical stress value is 241.68 MPa, and the error is only 7.6%, which verifies the feasibility of this analysis method. The fatigue life of four materials was analyzed, and it was finally obtained that the fatigue life of Ni–Cr–Mo steel SAE4340-350B was 1 × 1011, which was the best material. An optimization method based on the theoretical formula of helical gear bending fatigue is proposed, and the optimized model is tested by finite element software ANSYS, and it is concluded that the larger the modulus and tooth width, the better the fatigue life under certain conditions.