Abstract: To address the complexity of the fault system and the hydrocarbon accumulation mechanism in the Maichen Sag of the Beibuwan Basin, this study systematically investigated fault development characteristics and tectonic evolution patterns using seismic data, drilling data, and balanced cross-section analysis. The 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 composite hydrocarbon accumulations. The fault system exhibits layered evolution characteristics: the lower fault system (N-E-trending), which formed during the Paleocene–Eocene, controlled the deposition of the Changliu and Liushagang Formations, whereas the upper fault system (near E-W-trending), 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 sag stage. Clockwise rotation of the stress field led to changes in fault orientation. Variations in fault activity intensity control the distribution of source rocks, reservoir body types, and hydrocarbon migration pathways. These results provide a geological basis for exploring slope fault-block traps in the western high belt of the sag.