Abstract: A method which combines HFSS software with designer high frequency circuit to co-simulate a electro-optical polymer modulator with a in-plane coplanar waveguide electrode is proposed in this paper.A bandwidth of 56 GHz electro-optic polymer modulator is designed.First,the effective refractive index of light and microwave is optimized to meet the velocity matching.Then,the microwave characteristics of the electrode system are analyzed by co-simulation method to meet the impendent matching and lower microwave propagating loss.The results show that the co-simulation method can improve computational efficiency.And it is contributive to optimize a variety structure of traveling-wave electrode.