Abstract: In order to better understand the process and mechanism of hydrocarbon accumulation, the source rocks in the Wenchang A Sag of the Pearl River Mouth Basin is studied in this paper. Basin modeling was carried out to reconstruct the thermal evolution history of the study area based on the distribution pattern of source rocks and the abundance, type and maturity of organic matters. Data show that each source rock in the Wenchang A Sag is widely distributed with high abundance of organic matter and large thickness. The organic matter of the Wenchang Formation belongs to type Ⅱ₁, but the organic matter of the Enping Formation is type Ⅲ, and the source rocks are moderately or over matured. The source rocks entered into the oil threshold earlier in the inner part, but later in the outer part of the Sag, which remains in the low maturity today. The difference in oil and gas distribution pattern in the Wenchang A Sag is constrained by the differences in both the type and the maturity of organic matters. The inner part of the sag should be rich in gas, and the edge of the sag is the prospecting area for oil accumulation.