A newly modificated silicon (Si) avalanche photodetector (APD) desinged by standard complementary metal-oxide-semiconductor(CMOS) process was proposed in this paper. The basic structure of the Si APD which was formed by N-well/P-substrate was modificated with a deep N well below space charge area, and a independent voltage was applied on the deep N well to minish the transit time of electron hole pairs. The diffusion velocity and the drifting velocity can be improved at the same time, therefore, the 3-dB bandwidth will increase. The device parameters of CMOS APD were calculated with theoretical analysis, and the performance of the CMOS APD was optimized with SILVACO simulation, including technology simulation and device simulation. The simulation results show that when the window size of designed APD is 20 μm ×20 μm and when biased at 16.3 V, the APD achieves avalanche gain of 20, the best responsivity of 0.47 A/W，the 3 dB bandwidth of 8.6 GHz.