Abstract: Based on the basic principle of longitudinal warping displacement function setting, this proposed approach selects a series of warping displacement functions that meet basic warping mode of the box girder. Then, in consideration of the effect of shear lag, shear deformation and rotational inertia, the energy variation principle is applied to establish the governing differential equations and corresponding natural boundary conditions, and thus the natural frequency equation is obtained for the box girder. Next, using the equations, the natural frequency is determined under the given warping displacement functions. Finally, according to the magnitude of the natural frequency, the precision of warping displacement function setting can be judged. In the static calculation examples, this study concludes the necessity of precision selection of warping displacement function. Meanwhile, the comparison is made for the finite shell element and the finite strip solutions with the analytical solutions to indicate the efficiency of the approach.