In order to reduce the leakage current and improve the reliability of GaN-based UV detectors, the selection of the passivation layer film system and the optimization of its process are crucial. In this paper, GaN-based metal-insulator-semiconductor (MIS) devices were prepared using the following passivation layers: Si3N4 grown by plasma-enhanced chemical vapor deposition (PECVD), Si3N4 and SiO2 grown by inductively coupled plasma chemical vapor deposition (ICPCVD), and Al2O3 grown by plasma atomic layer deposition (PEALD), respectively, to investigate the MIS The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of the devices were studied comparatively. It was found that using PECVD-grown Si3N4 as the passivation layer has a lower leakage current in GaN-based MIS devices; by introducing a layer of PEALD-grown Al2O3 into the double-layer PECVD-grown Si3N4 passivation layer, the interfacial state density was further reduced: the average interfacial state density decreased from 3.94×1013 eV-1cm-2 to 3.52×1011 eV-1cm-2 .Using this "sandwich structure" passivation film, a p-i-n type GaN-based UV avalanche detector was fabricated, and the dark current at 113 V reverse bias was reduced from 3.73×10-8 A to 3.34×10-8 A compared with the detector of the Si3N4 film system without PEALD-grown Al2O3.