Abstract: Three generalized displacement functions are employed in analyzing shear lag effect of the thin-walled girder’s web. The differential equations and the corresponding natural boundary conditions of the box girder are induced based on the minimum potential principle. According to the fundamental differential equations and the corresponding natural boundary conditions, the static and dynamic characteristics of thin-walled box girder’s webs are discussed. A comparison between the finite element solutions and the analytical solutions verifies the validity of the proposed approach.