Abstract: Three-dimensional beam propagation method based on the series expansion method(3D-SEM-BPM) transform the BPM into the first-order normal differential equations. The method has the merits of simple calculation and high calculation efficiency. The lithium niobate electro optic waveguide modulator with Mach-Zehnder structure is simulated with this method. We got its optical field distribution and acquired its optimal structure and its power loss curve. The function of tangent is applied to map the infinite space into a finite space to avoid boundary truncation.