Abstract: To address the complexity of the fault system and the hydrocarbon accumulation mechanism in the Maichen Sag of the Beibu Gulf Basin, we systematically investigated the fault development characteristics and tectonic evolution patterns using seismic data, drilling data, and balanced cross-section analysis. Results indicate that the sag is primarily characterized by extensional structures and an extensional-strike-slip composite structural style, where the “cabbage-like” structural pattern of the central fault-uplift belt controls the composite hydrocarbon accumulation. The fault system exhibits layered characteristics in evolution: the lower fault system in NE-trending that formed during the Paleocene-Eocene, controlled the deposition of the Changliu and Liushagang Formations, whereas the upper fault system in near E-W direction, which developed during the Oligocene, governed the deposition of the Weizhou Formation. The tectonic evolution underwent four stages: initial rifting (NW-SE extension), intense rifting (NNW-SSE extension), late rifting (near N-S extension), and a subsequent sagging stage. Clockwise rotation of the stress field led to the changes in fault orientation. Variations in fault activity intensity control the distribution of source rocks, reservoir body types, and hydrocarbon migration pathways. This study provided a geological basis for exploring slope fault-block traps in the western high zone of the sag.