Abstract: At present, some simplified algorithms are still used in the design of pile-raft composite foundation of high-speed railway in China because of the lack of a reasonable formula. But their results can not meet the accuracy requirements. The theoretical solution to the raft is essentially studied. In order to get the analytical solution to the raft of the pile-raft foundation, the acting area of a single pile on the raft is selected as an element for solution. The element is regarded as a Reissner rectangular moderately thick plate with four edges shear free-clamped on the Winkler elastic foundation on piles. During calculation, the basic elasticity equation for Reissner rectangular moderately thick plates is transferred into the Hamiltonian system. By separation of variables in Symplectic geometry and characteristic function, the analytic solutions to the bending problems of the rectangular plates with medium and thick thickness can be obtained, and then the internal force, displacement, counter-force at pile top and pressure between the soils and the piles can be calculated. By comparing the measured values with the present analytical solutions, it is shown that the calculated moment of the raft, axial force and pressure between the soils and the piles agree with the measured values. The proposed method is proved to be reliable and can be adopted for the design of the pile-raft composite foundation.