A semi-analytical solution theory is presented by combining the separable Hamilton canonical equation and finite element method. The problem of free-edge effect of orthotropic laminates is studied in symplectic system. First, the separable Hamilton canonical equations are established with modified Hellinger-Reissner variational principle. And the state equations are established with finite element method. Then, an example analysis of the distribution of interlaminar stresses in composite laminates is calculated. The numerical results show the efficiency of the proposed method.