Abstract: Calculations for three-dimensional optical waveguides are successfully simplified into two-dimensional ones by combining the effective index method (EIM) with the finite difference beam propagation method (FD-BPM),and the optical transmission field of Mach-Zehnder switches is calculated and analyzed by two-dimensional effective index-finite difference beam propagation method (EI-FD-BPM).The structure parameters (e.g.the cross section of channel waveguide,the switch structure and its size) of Mach-Zehnder waveguide switches are optimized and a waveguide switch with higher transmission efficiency is achieved. It is demonstrated that the use of a two-dimensional EI-FD-BPM is feasible and easier instead of a three-dimensional FD-BPM,which the result difference from the two methods for the small waveguide structure is within 1.5%,and the calculations are reduced greatly,thus save time effectively. The approach provides theoretical guide for the designing and fabricating optical waveguide device at a high accuracy.