Abstract: Combined with series expansion method (SEM), threedimensional scalar beam propagation method (BPM) is transformed into the firstorder normal differential equations. And the function of tangent is applied to map the infinite space into a finite space to avoid boundary truncation. Compared with the traditional BPM, this method has the advantages of simple calculation, high calculation efficiency and small deduced matrix. The lithium niobate electrooptic waveguide modulator with MachZehnder structure is simulated by this method. The authors get its optical field distribution and acquire its optimal structure and its power loss curve.