Abstract: For a twin-cell box girder, from the difference of the shear lag warping between different cantilever plates, and combined with the axial force equilibrium condition of the whole section, a new shear-lag warping displacement function is defined for each cantilever plate of the box girder. The governing differential equations of equilibrium for the shear lag for the twin-cell box girder are established on the basis of the variational principle. For a typical simply supported beam with a twin-cell box girder, using three dimensional numerical methods of the plate-shell element and the analytical solution method proposed in this paper, respectively, the shear lag distributions under the uniform load and the concentrated force are obtained. It is shown that the new displacement pattern of the shear lag warping can reflect the difference of the shear lag warping between different cantilever plates and the new analytical solutions are in good agreement with the finite Element results. The shear lag effect on the top and bottom plates near the middle webs is different from that near the side webs on the twin-cell box girders. For an example girder, the stresses near the middle webs are larger than those near the side webs.