Recently important breakthrough has been made in discovery of natural gas in the deepwater areas of Qiongdongnan Basin. There are some disputes on understanding of natural gas accumulation in deepwater area. Therefore, it is necessary to conduct systematic study for gas genetic type, source, accumulation course and model. Our study shows that L17 natural gas is dominated by hydrocarbon gas, with low CO2 content. Carbon isotope features of hydrocarbon gas and carbon dioxide show that hydrocarbon gas is coal-type gas high maturation with organic origin. High contents of isoparaffinic and cycloparaffinic hydrocarbons in C6 and C7 components of gas lighter hydrocarbons indicate that the parent of natural gas was mainly from terrigenous higher plant. According to comprehensive analysis of maturation history of Eocene and Oligocene source rock, natural gas of L17 deepwater giant gas field was derived from the Oligocene Yacheng Formation. Largescale confined mass transport deposit developed within the upper Miocene Huangliu Formation, forming favorable sandstone reservoir. Highdensity sandy mass transport deposit, which is sealed by neighboring abyssal mudstone and configurated by “west low and east high”structural framework, formed structurallithological traps. Faults and microfaults of Paleogene structural layer served as migration pathways which communicated source rock and reservoir. Natural gas generating from early Oligocene Yacheng Formation migrated up to the Huangliu Formation structurallithologic traps and accumulated, driven by pressure difference between source rock and reservoir. The natural gas accumulation model includes four key factors: Oligocene Yacheng Formation source rock, largescale sandy mass transport deposit reservoir, cap rock of abyssal mudstone and migration pathway composed of faults and microfaults. These four factors control natural gas accumulation jointly in the deepwater areas of Qiongdongnan Basin. Three favorable exploration trends presented in this study are middleMiocene Meishan Formation basin floor fan lithological traps, south slopeuplift reefs, and Paleogene shore Lingshui Formation fault traps.