This paper investigates the punch-through characteristics of separate absorption， charge and multiplication avalanche photodetector （SACM APD）. By analyzing the device''s spectral response， capacitance characteristics， and I-V characteristics at various operating temperatures， and combining these with simulated internal electric field and energy band distributions from the SILVACO platform， we analyzed examined the performance of the SACM APD before and after punch-through and established a corresponding mathematical model. Through structural and process parameter optimization for silicon-based SACM APD devices， simulations revealed that when the ion implantation energy of the field-control layer was 580 keV， the optimized device exhibited a punch-through threshold voltage of -30 V and a capacitance reduction to one-third of the pre-punch-through value. Subsequently， a silicon SACM APD device was fabricated using the complementary metal-oxide-semiconductor （CMOS） process. Measurements confirmed a punch-through threshold voltage of -30 V， a 2.18-fold increase in photocurrent at 808 nm （punch-through）， a redshift of the peak responsivity wavelength from 590 nm （pre-punch-through） to 820 nm （post-punch-through）， and an elevation of the peak responsivity from 0.171 A/W@590 nm to 0.377 A/W@820 nm. The capacitance was also reduced to one-third of the pre-punch-through value at 1 MHz.