Mesozoic sags of the Qiongdongnan Basin are prospective areas for oil and gas discovery. To make petroleum breakthroughs in Mesozoic sags in the Qiongdongnan Basin, it is urgent to clarify the conditions of hydrocarbon accumulation and the favorable areas for exploration in those Mesozoic sags. Based on geophysics, geochemistry, drilling and paleontology data, systematic research on Mesozoic sags in the Qiongdongnan Basin was carried out. The results show that Mesozoic sedimentary formations were distributed in the Yabei Sag, Yacheng Uplift, Yanan Uplift, Lingnan Uplift and Southern Massif in the Qiongdongnan Basin. According to the analysis of tectonic evolution, Mesozoic formations were formed in the back-arc extensional basin in the Yanshanian period with subduction of the Pacific Plate to the Eurasian Plate, which has the possibility of forming source rocks, reservoirs, and traps. Because the potential of Mesozoic sags is larger in the Yacheng-Yabei area of the northern depression than in other areas in the Qiongdongnan Basin, this paper focuses on the comprehensive study of petroleum geology in the Yacheng-Yabei area that includes the definition of the Mesozoic formation and defines the favorable area and prospects of Mesozoic rocks. This paper further notes that Mesozoic lacustrine source rocks, delta reservoirs, structural and stratigraphic traps developed in the Yacheng-Yabei area. The southern slopes of the western sag and central massif are two favorable areas for exploration. To expand exploration in these areas, we hope to make petroleum breakthroughs in Mesozoic formations.