Abstract: The additional deflection induced by shear lag effect is adopted as the generalized displacement of a thin-walled box girder. Based on the new generalized moment and warping displacement function defined, the shear lag deformation state is separated from the flexural deformation state of an elementary beam and analyzed as a fundamental deformation state. Two modification factors are introduced in the generalized warping displacement function to consider fully the self-equilibrium of shear lag warping stresses. A simple and convenient formula for shear lag warping stress is proposed, which has the same form as that of the bending stress of an elementary beam. The governing differential equation for shear lag is established by using energy calculus and its initial parameter solution is given, in which the generalized moment, generalized shear force, additional deflection and its variation rate are chosen as the initial parameters. A two-span continuous box girder model is analyzed and the calculated results are in a good agreement with test results and those provided in other reference literatures. The shear lag effect increases the mid-span deflection of a box girder under concentrated and uniformly distributed loads by 17% and 16%, respectively.