InAlAs/InAlGaAs cascade multiplication avalanche photodiode was fabricated experimentally. The dark current of the diode was studied and analyzed with the change of the diameter and temperature of the mesa. The components of the dark current were calculated by MATLAB combined with the dark current function model. The effects of defect concentration N_t and surface recombination rate Sof the chip structure on the dark current were simulated and studied. The results show that the dark current of the diode mainly comes from the body dark current, not from the surface leakage current. The defect-assisted tunneling current I_tat affected by defect concentration dominates the dark current at 90V bias. The defect concentration in the chip structure N_t is 10¹⁹m-3 and the defect concentration in the absorption region NInGaAs is 7×10¹⁵m-3. Because the defects mainly come from the InAlAs/InAlGaAs multiplication region and the absorption region, and the proportion of the defects in the absorption region is very small, so it believes that the defects mainly come from the heterojunction InAlAs/InAlGaAs multiplication region.