Abstract:A complete symplectic eigenfunction expansion of rectangular moderately thick plate problem with two simply-supported opposite sides is proposed by using Fourier analysis method. The eigenfunctions of the Hamiltonian operator arising from the considering problem are directly solved with the help of Mathematica software. The completeness of the eigenfunctions is then proved, providing the theoretical guarantee of using separation of variables to solve the problems. The analytic solution is obtained with the proved expansion theorem. A numerical example shows the correctness of the result.