Abstract: Research and exploration in recent years show that some marginal sags also hold good hydrocarbon accumulation conditions and exploration potential. The Pearl River Mouth Basin, northern South China Sea, have many marginal sags in low exploration degree, and it is of great significance to systematically sort out the structure, tectonic evolution, and accumulation conditions of these marginal sags. Taking the Xijiang 36 Sag as an example, we analyzed the tectonic evolution, prototype basin restoration, seismic facies comparison, and oil and gas migration using 3D seismic, well logging, and geochemistry data, based on which the structure, hydrocarbon source potential, and accumulation characteristics of the Xijiang 36 Sag were systematically researched. Results show that (1) Xijiang 36 Sag was a half-graben rift lake basin controlled by an NE-striking fault in the south and structural overlapping in the north; (2) Xijiang 36 Sag holds a certain scale of medium-deep lacustrine facies source rock, and is mature with good hydrocarbon accumulation conditions; (3) affected by the regional mudstone caprock and the fault throw of the source fault, Xijiang 36 Sag was characterized by deep Paleogene near-source accumulation. Therefore, we suggest the Paleogene structure, stratigraphy, and lithologic trap are the main target for further exploration. This research helps to promote the exploration process of the Xijiang 36 Sag and provides a reference for oil and gas exploration in other marginal sags in the Pearl River Mouth Basin.