Aviation spur gear was taken as the research object, the influence law of rim thickness on root stress was discussed by using 3D finite element method. The variation trend of rim influence factor with thickness in ISO6336/AGMA standard was introduced. The fivetooth local model was established by using a 3D finite element method, the variation law of the rim influence factor was discussed, and the results were compared with the standard and literature to prove the correctness of the model. On this basis, the 3D finite element model of the whole spur gear was built by comprehensively considering the structural features ,such as thin rim, thin web and flexible shaft. The influence laws of rim thickness on the tooth root stress and the rim stress were analyzed in detail. The applicability of ISO6336/AGMA standard in the bending strength rating of the aerospace spur gear was discussed