Abstract: Based on the governing differential equations by variational principle, a finite element method for analyzing shear lag effect of box girder is proposed. The shear lag coefficient matrix and the generalized load vector are obtained. In addition, a procedure for assembling structural shear lag coefficient matrix and generalized load vector is demonstrated according to the boundary condition and the continuous condition of generalized shear lag displacements in structural system analysis. The shear lag effects of continuous and cantilever box beams, which have different boundary conditions, are analyzed under different loads. It is shown that the results obtained from the finite element method agree well with those from analytical method. The reliability and wide applicability of proposed method are also verified by the examples.