Steel-concrete composite beams are composed of concrete slabs and steel girders by shear connectos. Due to the shear lag effect, the longitudinal normal stress of cross section is nonuniform distribution,and it is difficult to analyse the effect of that by ordinary beam theory. A differential equation of equilibrium is constituted corresponding to the compatibility of deformation and the equilibrium of forces about steel-concrete composite beams under particular assumed condition. The method of variable-separating is applied to solve the differential equation with the simply supported boundary condition. An example of steel-concrete composite box girder is given to analyse the effect of shear lag on its stress and approve its applicability.