Abstract: During the production of heavy-duty gears, the surface is hardened to improve the overall performance. However, most of the existing analysis methods regard the gear surface as homogeneous material, without considering the influence of the hardened layer. In this paper, a gear solid modeling method is proposed, in which the hardened layer of gear surface is stratified and the material property changes gradually in the form of the power function. Based on the Hertz contact theory, a finite element numerical model is established to study the influence of the hardened layer on the stress field of the heavy-duty gear. The results are as follows: The meshing stress field is closer to the theoretical solution through the modeling method proposed in this paper; reasonable distribution indexes of the hardened layer can not only effectively reduce the stress of the gear but also increase the bonding strength; the thickness of hardened layer has a great impact on the contact stress, and when the thickness is between 1 mm and 1.5 mm, the stress field distribution of the gear is the most reasonable.