Ge/Si Separate-Absorption-Charge-Multiplication(SACM)-APD, as a new type of silicon APD, has become the focus of research. The device structure and its main parameters of Ge/Si SACM-APD(including quantum efficiency, responsivity, dark current, etc) were investigated in detail from the theory analysis and simulation. The simulation results show that the avalanche breakdown voltage is 25.7 V, the internal quantum efficiency is 90%, the maximum responsibility is up to 55 A/W when the gain is 1. The device is most sensitive in the spectral range of 750-1 500 nm. The peak wavelength of the APD is 1 050 nm. Under the condition of high bias and high light intensity, the electric field profile can be affected through the space charge of these electrons and holes.