In order to study dynamic characteristics of a thin walled box girder, considering the shear deformation and shear lag effects, the energy variation pr inciple was used to establish the governing differential equations and the co rresponding natural boundary conditions about the three generalized dynamic disp lacement functions w(x,t),u(x,t),θ(x,t). The closed form solutions to the g ener alized displacement functions were obtained which discovered the dynamic law of the thin walled box girder. An example is analyzed to compare the finite shell element solutions with the analytical solutions, the proposed approach is testified.