Abstract: In this paper, the shear lag and shear deformation effects are analyzed with a simple and efficient method, considering three longitudinal displacement differential functions for trapezoidal section box girder with varying depth. The governing differential equations and boundary conditions of the box girder under lateral loading are derived using energy-variational principle. The closed-form solutions of the equations are obtained and compared with the finite strip solutions. Good agreement is reached. The influences of three longitudinal displacement differential functions upon the shear lag are also discussed. It is shown that the present theory is effective and general.