Abstract: The finite element method employed in a plane bar-system is usually applied in the structural analysis of construction simulation for the bridge with a composite deck; however, it will produce significant errors if the shear lag effect within slabs is neglected. Taking into account the long-term behavior of the concrete material, the shear lag effect within slabs of a composite deck is considered by means of a shear warping shape function and an intensity function. As two unknown variables, which are the vertical displacement of the deck and the intensity of warping, a variational balance condition is derived by the virtual work theorem. As a result, the shear-lag effect is expressed as “an additional bending moment”. Analytic solution for a specific structural form with relevant boundary conditions can be conducted. This method is verified by the finite element method correctly and applied to the investigation of the long-term behavior of a composite deck with two fixed end constrains. The results obtained from the code based on the effective flange width are intervenient between those obtained from the elastic and viscoelastic model. The errors of a plane finite element model considering shear lag effect can be minimized by the method proposed in this paper.