Abstract: The shear lag effect in steel-concrete composite beam structures with wide flanges leads to longitudinal buckling deformation of the concrete slab and the bottom and web plates of the steel girder, and has significant effect on stress and deflection in composite beams. In order to further analyze its mechanism, the governing differential equation of a double-box composite structure with wide flanges considering the shear lag effect is deduced, based on the minimum potential energy principle, and ignoring slip effects in the interface, under the assumption that longitudinal deformations of the concrete slab and steel girder bottom-plate follow an approximately parabolic distribution along their width. Then analytical expressions of stress and deformation are obtained for a beam with both ends simply supported, under concentrated loading. Finally, a comparative analysis between a numerical example, composite beam experiment, and finite element simulation is implemented, and the results show that the proposed approach provides a theorical basis for analyzing the shear lag effect in composite beam structures, and this method has a definite guidance and reference value for engineering applications.