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 paper, the Huaguang Sag was selected as a case for study. The study reveals that there are four types of extensional structures in the area, namely graben, half graban, terraced graben and terraced half graben, formed in two stages, corresponding to the two regional structural evaluation stages in the region.   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.