Abstract: In this paper the beam functions of short beam with the uniform load or a concentrated load is given.The system of basis function for rectangular elastic middle-thich plates is built by the short beam functions.Using the theorem of minimum potential energy we can obtain the approximate solution of polynomial form for bending problems of rectargula elastic middle-thich plates under arbitrary load.The calculations show that the method in this paper has higher prec ision.When the shear rigidity C→∞, the result in this paper is coincided with the result of 5.