Abstract: Indium antimonide (InSb)-based avalanche photodiodes(APD) are widely used in infrared detection because of their high sensitivity, stability, and low cost. However, the narrow bandgap of InSb at room temperature introduces significant noise mechanisms, which degrade the device performance. To address these issues, based on the conventional separate absorption and multiplication (SAM) structure, a charge layer was introduced, and a novel heterostructure-selective avalanche charge modulation structure, termed separate absorption charge and multiplication(SACM), was proposed. A two-dimensional back-illuminated multiplication SACM-type photodetector model was constructed using the Silvaco ATLAS simulation platform. The effects of doping concentration and layer thickness on device performance were systematically investigated. The results show that with a charge layer doping concentration of 5 × 10¹⁶ cm^-3 and thickness of 0.1 μm, the avalanche gain is maximized and the dark current is minimized. Compared with the SAM structure, the SACM structure exhibited a higher maximum photocurrent. Its maximum responsivity reached 3.06 A/W at a wavelength of 5.6 μm, which is 1.4 times that of the SAM structure. The external quantum efficiency at the peak wavelength increased by 20%, reaching 67.6%. Moreover, the normalized detectivity is improved by 65.5% compared with that of the SAM structure.