It is well known that shear lag effect exists in wide flange concrete slabs. Traditionally, the effective flange width method is employed for considering the shear lag effect in a steel/concrete composite beam. The limitation of this method is that the load types and constraint conditions are generally ignored. In this paper, an analytical method using full flange width has been proposed by introducing shear warping shape function and intensity function of the shear warping displacement. By mean of two unknown functions: the vertical displacement of the whole cross section and the intensity of the warping, a variational balance condition which involves equilibrium equations with relevant boundary condition is imposed by the virtual work theorem. As a result, the shear-lag effect is considered by additional virtual external load, expressed as “additional bending moment”. The performance of this method dealing with elastic problem has been verified by finite element method using solid elements. Finally, the method is applied to investigate the long-term behavior of a beam fixed at two ends. It has been found that the results yielded from the code based on the effective flange width method are intervenient between those obtained from the current model with elastic and viscoelastic problem; the result errors of beam finite element model considering shear-lag effect can be modified by the method proposed in this paper.