Abstract: The Hu Hai-Chang's12 differential equations of the elastic middle-thick plates and its boundary condition are applied in this paper. Taking δ=D/(Ca²) as a small parameter, we change the boundary value problem with potential function F of generalized displacement into a series of bending problems in classical theory of plates by the perturbation method. Thus we obatin the asymptotic solution for F. The principle of minimum potential energy is created to solve the bondary value problem of the boundary effect function f. Caeulation examples show that methods in this paper have higher accuracy for the approximete solutions of generalized displacement of elastic middle-thick plates.