By means of Hamiltonian formulation of PDEs and related two-variable variational principle combined with finite element method, the authors put forward a group of Haliltonian-system-based semi-analytic methods. The formulations of two typical semi-analytic methods of such group are presented. Numerical example by one of the methods is given finally. It is found that the new semi-analytic method can lead to rather good results compared with analytic method, Ritz method, finite strip method and finite element method.