An analytic model is proposed to study the linearity performance of the silicon Mach-Zehnder optical modulator. According to the simulation results, we optimize the width of the silicon rib waveguide and the location of the PN junction to improve the linearity performance. The fabricated silicon Mach-Zehnder optical modulator has a spurious free dynamic range of 113.3 dB.Hz2/3 and 88.9 dB.Hz1/2 for the third-order intermodulation distortion and the second-order harmonic distortion. We also demonstrate the optical four-level pulse-amplitude-modulation (PAM-4) signal generation through the device. The generated optical PAM-4 signal is characterized at the rates up to 35 Gbaud. The BERs of the optical PAM-4 signals can reach 5.2╳10-6 at 20 Gbaud and 6.6╳10-5 at 32 Gbaud, which are much lower than the threshold of hard decision forward error correction (3.8 ╳10-3).