In this paper, a graphene M-Z electro-optical modulator based on S-Y branch SOI ridge waveguide was designed by COMSOL, and the influence of the Y-branch waveguide curvature radius, the length of the phase arm, core spacing and input-output straight waveguide length on the modulator were analyzed. On this basis, the performance index of the electro-optical modulator was calculated and optimized. Finally, a silicon graphene M-Z electro-optical modulator with a 3dB bandwidth of 143GHz and an extinction ratio of 15.05dB was designed.