A finite element method considering interaction betwee bending and shear lag deformation was proposed and the finite element formulations including the effect of shear lag was deduced. The effect of shear lag on structural behaviours in indeterminate boxgirders was studied in detail based on the proposed method. Firstly, the shear lag effects on deflection and the shear lag coefficients were analyzed for both determinate and indeterminate beams. The results obtained by using the proposed method were compared with those by using the analytical method. Finally, the effect of shear lag on redistribution of internal forces including shear force, bending moment, and additional bending moment due to shear lag in indeterminate boxgirder bridges was studied. It is found that the effects of shear lag on deflection and stresses at a crosssection are obvious for both determinate and indeterminate beams, while the effect of shear lag on internal forces in indeterminate boxgirder bridges is small and may be neglected.