In order to improve the S/N ratio, reduce the dark current and develop the solar-blind photodetectors with high performance, the dark currents of the AlGaN solar-blind ultraviolet avalanche photodetectors were studied. Firstly, the conventional p-i-n-i-n avalanche photodetectors were examined using two newly designed models of avalanche photodetectors in GaN and AlGaN. Different characteristics of dark current were analyzed and the simulation of the dark current matched the experimental data well. Based on this, the effects of the negative polarization charge at different heterogeneous interfaces, the p-type effective doping and temperature on the dark current in high Al content AlGaN solar-blind avalanche photodetectors were investigated in detail. Results show that the dark current of the AlGaN solar-blind ultraviolet avalanche photodetector is about 2.5×10-13 A near zero bias, the avalanche breakdown occurs at about 138 V in reverse and the open current is about 18.3 nA, temperature coefficient for the breakdown voltage is about 0.05 V/K, which are all consistent with the experiments and the literature results well.