To simulate the stress characteristics of web gap more accurately in steel plate girder bridge, a global model of a four-span continuous steel plate composite girder bridge was established. Considering various structure of main girder, the local deformations of web gap were analyzed. Based on the deformation characteristics, the simplified structural mechanics model was established, and the correctness of the model was verified by the stress test data in real bridge and finite element calculation results. Considering the small length and large thickness of web gap, the shear stress and normal stress of web gap of web gap were further determined by using this model. The results show that the web gap is affected by the coaction of "rotation" and "displacement". It would be more consistent with the actual stress characteristics when the web gap was simplified as the beam consolidated at both ends, and subjected to the coaction of rotation" and displacement. The ratio of shear stress and normal stress at the end of web gap is independent of the load, related to the length and thickness of the structure. The conclusions could provide a basis for multi-axis evaluation of web gap.