Abstract: Mach-Zehnder optical waveguide is an important component unit in electrooptic polymer modulators. By using effective index method (EIM) to calculate the transverse refractive index distribution of polymeric ridge waveguide, 3D light wave field propagation is successfully simplified into 2D propagation. Proceeding from the scalar wave equation observed by the guided modes, the basic computing scheme of the finite difference beam propagation method is obtained by utilizing the transparent boundary condition (TBC) and the CrankNicholson difference scheme. The effect of structure parameters such as ridge width and branch angle on the loss was theoretically studied with the support of the theoretical analysis of the TM mode light wave propagation and loss characteristics in the MZ waveguide structure. The combinational approach of EIM and 2DBPM demonstrates that the design of MZ waveguide can be made effectively by evaluating the influence of the structural parameters on the loss characteristics. The approach has high precision, small calculation complexity and high efficiency, and it provides a theoretical basis for the fabrication of electrooptical polymer modulators.