The GaAs/GaAlAs traveling wave directional coupler optical modulator on S.I. GaAs substrates with a n + epilayer and the coplanar microstrip electrode are designed based on theoretical analysis and especially the analysis and study of the microwave characteristics of the traveling wave electrodes in the device. Then, the problem of high transmission loss of the microwave in traveling wave electrode, which was found during the experiments, is analyzed and the related modified structure of the traveling-wave directional coupler modulator with a 35 GHz bandiwdth is designed.