Abstract: In order to better understand the process and mechanism of hydrocarbon accumulation and identify the source rock potential of Wenchang-A sag in Pearl River mouth basin, this study uses the technique of basin modeling to reconstruct the thermal evolution history in the study area based on the analysis of source rock distribution, organic matter abundance, types and maturation. The result shows that each layer of source rock of the Wenchang-A sag is widespread, with a high organic matter abundance and a large thickness. The organic matter of Wenchang Formation is of type Ⅱ1; while that of Enping Formation, type Ⅲ. The source rocks are in the stages of middle maturity and over maturity. Inside the Wenchang-A sag, the source rocks reached oil threshold in earlier time with a high maturity. However, on the edge of the sag, they reached oil threshold later, and is still in low maturation stage. The distribution of oil and gas in the sag is constrained by the different organic matter types and maturities. The pools of oil and gas come from Wenchang and Enping formations. The inside of the sag is a priority of gas searching, and the edge of the sag is favorable for petroleum.