Width to height ratio Π shaped girder of cable stayed bridge is very big, and the shear lag problem is very outstanding. For considering the variation of load and boundary conditions in construction process, a beam element with two shear-lag degrees of freedom at each node is established to take the coupling effect of shear lag and bending deformation of composite girder into account, which is suitable for the complex load and boundary conditions. In the formula derivation, it is assumed that the longitudinal displacements of the concrete slab are described by a third-power parabolic function, and the steel flanges share the same displacements with concrete slab at the same abscissa. The shear-lag effects of Πshaped composite girder in a cable-stayed bridge during the construction process is analyzed by using the proposed method. The dead weight of decks, tensioning loads and secondary dead load are considered in analysis according to the dynamic shear-lag boundary conditions during the construction process. The numerical analysis results were compared with measured ones. It was shown that the shear lag of main girder alternates with positive and negative effect along with the changes of load increments and boundary conditions in different construction stages. Comparing with the vertical loads such as the dead weight of decks, the cable-tensioning loads have a much more obvious effect on the shear lag of the main beam. The shear lag effect at the sections near the towers is more obvious than that at the other sections. As the construction continuing of the bridge, the shear lag coefficients of the Πshaped composite beam are gradually decreasing. In addition, it should pay more attention to the shear lag effect in the cantilever construction stages than that in the final structure.