Silicon photonics is a novel technique for high scale optic/electronic integration, with which most kinds of optical devices can be designed. Based on free carrier plasma dispersion, silicon Mach-Zehnder electro-optic modulator (MZM) can achieve high modulation speed over 10 GHz. For silicon MZM, one of the critical techniques is how to effectively change the free carrier concentration in order to obtain the needed modification of the optical refractive index. Considering the need for modulation speed and power consumption, a kind of silicon MZM based on metal-oxide-semiconductor(MOS) capacitor electrodes is studied by a simulation method combining electric and optic analytic mode. The results show that under the driving voltage of -1.5 V, when the doping concentration is 1015 cm-3, the effective refractive index change of the optical waveguide in silicon MZM is about 3.82×10-6, which result in the Vπ length of 13.5 cm and the loss of less than 0.04 dB/cm. When doping concentration is 1018 cm-3, the effective refractive index change can reach 6.96×10-5, which result in the Vπ length of 0.7 cm and the loss of about 24.8 dB/cm. Varied capacitor electrode structures of silicon MZM may make the visible difference of the modulation performance, and its effective refractive index change may reach 1.53×10-5, which result in the Vπ length of 3.3 cm and the loss of about 0.09 dB/cm.