Abstract: A great number of Cenozoic petroleum basins have been discovered in the northern continental margin of the South China Sea. Two of them, the Qiongdongnan Basin and the Pearl River Mouth Basin are located in the deepwater area of the region. Recently medium and large petroleum discoveries have been obtained in the southern deepwater area of the Pearl River Mouth Basin. It verifies that favorable petroleum geological conditions occur in the deepwater area. Having similar tectonic and depositional evolutionary history with the Pearl River Mouth Basin,the southern deepwater area of the Qiongdongnan Basin should have the same potential to form medium to large gas reservoirs. It, therefore, has become a necessity to study this area in details. The accumulation of high resolution seismic data in recent years has made it possible to understand more about the structural pattern of the deepwater area of the Qiongdongnan Basin. In this regard, the Huaguang Sag was selected as a case for study. Our study reveals that there are four types of extensional structures in the area, namely graben, half graban, terraced garaben and terraced half graben, formed in two stages, corresponding to the two regional structural evaluation stages in the region. There is an obvious double-layer vertical architecture in the sag, including the rifting lower layer and the subsiding upper layer, and four units have been recognized in the sag according to the structural pattern, namely the western sag, eastern sag, middle horst and the southern slope. The distribution of Paleogene deposits or the lower layer is obviously controlled by rifting. The tectonic activities in Neogene or the upper layer were rather weak. There is little tectonic deformation. The double layer architecture has provided an excellent combination for hydrocarbon accumulation.