Abstract: In order to analyze the influence of shear effect on mechanical characteristics of thin-walled box girders, the paper uses the shear equilibrium differential equation of microplate to derive the bending displacement function for thin-walled box girders, which accounts for the in-plane shear effect of girder plates. The additional deflection induced by shear effect is adopted as the generalized displacement of a thin-walled box girder. By defining the generalized moment and warping displacement function, the shear deformation state is separated from the whole flexural deformation state and treated as a fundamental deformation state. Considering the self-equilibrium of shear warping stress, the paper introduces two sectional characteristic parameters in the generalized warping displacement function. The numerical example of a two-span continuous box girder shows that the stresses at the mid-span cross section calculated by applying the shearing and bending displacement function are in a good agreement with test results and those by finite element method. The shear effect increases the mid-span deflection of the box girder under concentrated and uniformly distributed loads by 27% and 24%, respectively.