Abstract: The Changchang Sag is located in the Eastern part of the central depression of the Qiongdongnan Basin. It is a new deep water exploration target. Analysis of reservoir forming conditions indicates that there is sufficient hydrocarbon supply, with deltaic sandstone, coastal sandstone, submarine fan and channel filling as reservoirs, and well developed multilayer structural traps of larger size, that forms an excellent reservoir-cap rock assemblage for oil and gas accumulation. Assessment of resource potential for the five sub-sags of A,B,C,D,E of the Changchang Sag suggests that there occurs great oil and gas potential in the sub-sags of A. The large and medium-sized structural and lithologic traps of the WN X-1, WN X-2, CC X-1 and others should be favorable large targets for future exploration in the Qiongdongnan basin.