The upper and lower surfaces of the double layer structure are mechanically free. The electrical boundary conditions are electrically open, electrically short and connected with the air medium. The general solution of the SH waves are obtained by the constitutive equations of the PMN-PT piezoelectric materials. The dispersion curves are affected by the boundary conditions and the thickness ratio of the structure. The obtained results show that the propagation properties can be achieved in the processing of the boundary and the design of the structure.