Abstract: Double-system analysis method for local stress of steel box girder is presented based on the error analysis of stress calculation of the girder for Runyang Cable-stayed Bridge. According to the configuration characteristics of the steel box girder, the full-scale structural model and local-scale member model are constructed respectively and submodeling method is employed for the connection of these two models. The local stress responses of the girder under vehicle loads are calculated and compared with the experimental results to verify the effectiveness of the method, on the basis of which the mechanical characteristics of the girder under vehicle loads are further summarized systematically.