Based on Hu’s equations of moderately thick rectangular plates, the free vibration problem for the plates were transferred into Hamilton system. Then, the whole state variables were separated. Using the method of eigenfunction expansion in the symplectic geometry, the exact free vibration solutions of the plates with two opposite sides simply supported were obtained. Since only the basic elasticity equations of the plates were used, the method eliminates the need to pre-determine any trial functions and is hence more reasonable than the conventional semi-inverse methods. Numerical results were presented to demonstrate the validity and accuracy of the approach.