In this paper, the punch-through phenomenon was studied, based on a fabricated separate absorption, charge, and multiplication avalanche photodetector (SACM APD). The spectral response, capacitance characteristics, and I-V characteristics at different operating temperatures of the APD were measured and analyzed. Meanwhile, the device performance before and after the punch-through phenomenon were compared, and the model of the electric field region formed by external voltage was analyzed, based on the measurement results and the simulated the electric field and energy band distributions by the SILVACO. When the ion implantation energy of the charge layer was 580keV, the simulated device has a punch-through voltage of -30V and a capacitance reduction of one-third before punch-through. Then a Si SACM APD was prepared based on CMOS process. The punch-through voltage of the device was -30V and the capacitance was reduced to 1/3@1MHz before punch-through, which exactly match the simulation results. Moreover, the photocurrent after punch-through increases to 2.18 times of the before value at 808nm. The peak responsivity increases from 0.171A/W@590nm to 0.377A/W@820nm.