The modulation asymmetry of a multifunction integrated optic circuit (MIOC) can be characterized by the difference in the phase response to modulation electric fields with the same magnitude but different directions. The X-direction vertical component of the modulated electric field is one of the main reasons for the phase difference in this modulation. The difference in modulation phase during equal amplitude reverse modulation of the modulator introduces nonlinear phase errors in the fiber optic gyroscope optical path, which are difficult to directly suppress using phase compensation. This article proposes a sinking-electrode design for suppressing modulation asymmetry in modulators, which simultaneously improves the modulation efficiency of the modulator and significantly reduces the half-wave voltage of the modulator. By establishing a finite element simulation model, electrode parameters that meet application requirements were designed, the manufacturing process tolerance of the sinking-electrode modulator was analyzed, and etching experiments were conducted for verification. Compared with traditional modulators, modulators based on sinking electrodes have reduced the phase error of equal amplitude reverse modulation from 5×10^-4 rad to 1×10^-6 rad, and the maximum half-wave voltage can be reduced by 18%. The modulation symmetry and efficiency of the modulator are greatly improved, which is conducive to the development of high-performance fiber optic gyroscopes.