| In order to accurately analyze the vertical bending behavior of Twin-Cell composite box girder with corrugated steel webs, the accordion effect, the Self-stress equilibrium for the shear lag warping stress, the shear deformation and other factors are considered, three longitudinal displacement difference functions are employed to exactly describe the variation of shear lag in composite box girders with different widths and thicknesses of wing slabs. Next, based on the energy-variation method, the differential equations and the corresponding natural boundary conditions of the composite box girders are induced, and the contribution of accordion effect, shear lag effect and shear deformation to stress of the composite box girder is analyzed in detail. So, the example shows:(1)The accordion effect has a certain influence on the mechanical properties of composite box girder, and the accordion effect of composite box girder is more prominent under concentrated load.(2)The shear lag effect of composite box girder is obvious under simply supported boundary conditions, especially for composite box girder with concentrated load. (3)This paper presents a more refined analysis method on the mechanical properties of composite box girders, and research results in this paper have some guiding significance for the design of composite structures.