Abstract: in this paper,based on the author's previous studies,an important modification to the shear lag function for describing the widechannel beam so that both equilibriums of bending moment and axial force are satisfied has been given.By use of the principle of complementary energy, the control differential equations and the boundary conditions for the shear lag function are obtained.The formulas for calculating stresses and deformations in simply supported and cantilever beams under uniform load and concentrated unit load,and for calculating the effective width of the lower deck plate are also given.Finally,a numerical example is given so as to compare with the measured value and other calculating results.