The photoelectric characteristics of a GaAsSb/InAlAs separate absorption, charge, and multiplication (SACM) structure avalanche photodiodes (APDs) were investigated in this study. This heterojunction has a relatively small conduction band offset (=0.089eV), which facilitates easier injection of carriers into the multiplication region. Three different charge layer doping concentrations (2×1017cm-3,3×1017cm-3,4×1017cm-3) of SACM APD were designed. Experimental results indicate that the device with a charge layer doping concentration of 4×1017cm-3 exhibited the best performance. At room temperature, it achieves a cutoff wavelength of 1.7 , a punch-through voltage of -10.8 V, a gain of 20.3 at a bias voltage of -34.6 V, and a corresponding dark current density of 0.058 A/cm2. Compared with the GaAsSb PIN APD device, the gain is improved by 6.8 times, and the dark current is effectively reduced. By adjusting the doping concentration of the charge layer in the SACM APD, the dark current is suppressed, and the gain is improved, providing a new approach for highly sensitive short-wave infrared APDs.