We optimize the linearity performance of silicon carrier-depletion Mach-Zehnder optical modulator through controlling the doping concentration. The optical field distribution in the waveguide is a Gaussian-like distribution. As the doping concentration increases, the dynamic depletion width of the PN junction under the same modulation signal will decrease, and the integration width of the overlap between the Gaussian-like optical field distribution and the depletion region will become smaller. Therefore the modulated signal has less nonlinear components. Our simulation results proved this analysis. We also fabricated different devices with different doping concentrations. By adopting a ten times doping concentration, the spurious free dynamic range (SFDR) for third-order intermodulation distortion (TID) increases from 109.2 dB.Hz2/3 to 113.7 dB.Hz2/3 and the SFDR for second harmonic distortion (SHD) increases from 87.6 dB.Hz1/2 to 97.5 dB.Hz1/2 at a driving frequency of 2 GHz. When the driving frequency is 20 GHz, the SFDRs for TID and SHD distortions are 110.3 dB.Hz2/3 and 96 dB.Hz1/2, respectively.