Silicon photonics is a novel technique for large-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. Considering the need for modulation speed and power consumption, a kind of silicon MZM based on MOS capacitor electrodes is studied by a simulation method combining electric and optic analytic model. The results show that under the driving voltage of -2 V, when the doping concentration is 1×1015 cm-3, the effective refractive index change of the optical waveguide in silicon MZM is about 1.05×10-5, which results in single modulation arm length of 3.68 cm to realize Vπ and the loss of less than 0.84 dB/cm. When doping concentration is 5×1015 cm-3, the effective refractive index change is only 0.86×10-5, which results in the Vπ length of 4.51 cm and the loss of about 1.36 dB/cm. And varied capacitor electrode structures of silicon MZM may make the visible difference of the modulation performance, including the rise and fall time.