In this article, the dark current dominant mechanism of longwave infrared detectors based on InAs/GaSb superlattice were analyzed by using the analytical current model of diode. Firstly, the I-V test of variable area diode were performed, which confirm the dark current of InAs/GaSb superlattice longwave detector passivated by anodic sulfidization and SiO₂ were mainly originated from bulk current, not from surface leakage current; Then, the dark current of InAs/GaSb longwave infrared detectors were fitted by the current model of diffusion current, generation-recombination current, direct tunnel current, trap-assisted tunnel current. The result indicate the dark current of detector was dominated by G-R current at low reverse bias (≤60 mV), while at high bias (>60 mV), the dark current is dominated by direct tunnel current. The effect of doping density of absorption layer on these two currents are analyzed, and confirm that the optimum doping density is 5×10¹⁵-1×10¹⁶ cm^-3.