Abstract: Reprocessed multi-channel seismic data collected from the Zengmu Basin is interpreted in this paper combined with gravity and magnetic anomaly data. It is confirmed that the “Zengmu Xi Fault” is located in the southernmost part of the western-margin of the South China Sea in a nearly S-N direction. The structural characteristics of the fault, especially its composition and spatial framework, are carefully described and explained. It is found that the Zengmu Xi fault is composed of two faults, the Fault 1 and Fault 2. The Fault 1 is located at 4°30′N～7°00′N, 109°30′～110°E, with a main tendency to ESE, and a strike towards NEN. It is a large boundary controlling fault of the Kangxi Depression, while the fault 2 is distributed between 3° and 5°N, 109°30′E and 110°30′E, with a main fault tendency to NE and a strike towards NNW. It is a large boundary controlling fault of the Tatao horst graben. The southward extension of Zengmu Xi fault 1 ends when it meets the fault 2. Upon the basis, the characteristics of the tectonic activity of the Zengmu Xi fault and its control over the depositional evolution of the Zengmu basin are analyzed and discussed.