Abstract: Three independent generalized displacement functions w(x),U(x),θ(x) are employed in analyzing shear lag effect of thin-walled beams with wide flange by calculus of variations.The fundamental differential equations in terms of w(x),U(x),θ(x) and corresponding boundary conditions are established by means of the principle of minimum potential energy.The work done by shear force on shearing deformation is taken into consideration when calculating general potential energy of the system.The fundamental equations and boundary conditions indicate that shear lag effect and shear deformation effect are independent.Numerical results of thin-walled beams with wide flange show that the accuracy of deflection is improved significantly.