The waveguide modulator with a ridge structure is analyzed by using the Fast Fourier Traslator method. According to the fact that the thickness of the SiO2 buffer layer can not be ignored, the impact due to the thickness of the SiO2 buffer layer is especially calculated. On the basis of this calculation, the impact of the SiO2 buffer layer thickness on the characteristic impedance and dielectric constant in the modulator with a ridge structure is obtained. After considering the factors such as speed matching, impedance matching, half-wave voltage and difficulty in fabrication, all parameters of the modulator are optimized. The results have shown that the modulator with a proper thickness of buffer layer can effectively reduce its equivalent refractive index while keeping larger characteristic impedance, thus greatly improve its modulation width and meet the requirement of high speed fiber optical communication.