For avalanche photodiodes （APDs）, low operation voltage is required for integrated circuit stability and low power consumption. In this paper, a model for InAlAsSb separate absorption, charge, and multiplication （SACM） APD is established. To get higher gain at lower reversed bias voltage without sacrificing the operating voltage range, a high/low band gap multiplication layer is adopted. The effects of the thickness and doping concentration of the multiplication layer on the dark-current and the break-down voltage have been investigated. By optimization of the doping concentration, the break-down voltage and punch-through voltage can be decreased simultaneously.