Abstract: On the basis of previous researches, fine interpretation is made for the fault systems of the Wenchang A Sag with the data of 3D high-resolution seismic survey, coherence attribute analysis and other techniques. The results show that the basin is characterized by typical extensional structures, such as plow-like structures, flower-like structures and rotating normal faults. Affected by the Zhu-3 south fault, the southern boundary of the fault system shows a step-like pattern. During the period from Late Eocene to Early Oligocene, EW faults were developed under the subduction and retreat of the Pacific Plate, the collision with the Indo-subduction plate, and the southward subduction of the ancient South China Sea. During the period of Late Oligocene, however, under the influence of the South China Sea expansion event, the early dextral stress field was strengthened, while formed the nearly EW fault, and then evolved to those in NWW trending. From Late Eocene to middle Miocene, the fault trend rotates clockwise in the direction from NE to EW to NWW under the action of continuous right-hand tensional stress field. The tensile strength gradually weakened later on. Data suggests that the evolution and genetic mechanism of the faults in the Wenchang A Sag are consistent with the variation in stress field on the Northern Continental Margin of the South China Sea. This study is conducive to further understanding the structural characteristics and evolution of the oil-bearing basins on the northern margin of the South China Sea and improving the efficiency of oil and gas exploration and development.