Abstract: An analytical model is proposed for steel-concrete composite beams accounting for not only the slip between the steel beam and concrete slab but also the shear lag effect in the concrete slab. In order to considering the shear lag effect, a warping shape function accounting for the non-uniform transverse distribution of longitudinal displacements in the concrete flange is introduced into Gjelsvik’s model. Then analytical solutions for the cases of simply supported composite beam under uniform distribution load and cantilever composite beam under end concentrated load are derived. From the comparison of the results of FEM and analytical solutions, it is shown that the shear lag effect in composite slab and slip effect in the interface are coupled. The shear lag effect decrease when reducing shear stiffness of interface, while slip deformation and deflection of composite beam increase. The thickness of concrete slab has little impact on the shear lag effect. It is also indicated that there is apparently difference between the distributions of effective width coefficient along the beam axis under different load type.