In recent years, separate absorption, charge and multiplication avalanche photodiodes (SACM-APDs), including InP/InGaAs, InAlAs/InGaAs and Si/Ge APD, have drawn a lot of attention in the field of optical communication. In this paper, on the basis of the circuit model, the frequency response is studied systematically for APDs with different thicknesses of epitaxial layers, different multiplication materials and device structures. The effects of the absorption layer thickness, the dimension of the active area and the parasitic parameters on frequency response are addressed to Si/Ge APD. The simulation resuets are in good agreement with the experimental results, which indicates that the circuit model is helpful for the design optimization of APDs.