Abstract: Nan′an Basin is a giant hydrocarbon basin, but its tectonic division scheme and associated fault systems has not been well understood. Based on newly acquired seismic data from the southwestern margin of the South China Sea, this study analyzed the structural units, tectonic feature and geodynamics of the sedimentary basin. The new data suggests that the Nan′an Basin is a rift basin oriented in the NE-SW direction, rather than a pull-apart basin induced by strike-slip faults along the western margin. The basin is a continuation of the rifts in the southwest South China Sea since the late Cretaceous. It continued rifting until the middle Miocene, even though oceanic crust occurred in the Southwest Subbasin. However, it had no transfer surface at the end of spreading, where it was characterized by a late middle Miocene unconformity (reflector T3). The Nan′an Basin can be divided into eight structural units by a series of NE-striking faults. This study provides evidences to confirm the relative importance and interplay between regional strike-slips and orthogonal displacement during basin development and deformation. The NE-SW-striking dominant rift basin indicates that the geodynamic drivers of tectonic evolution in the western margin of the South China Sea did not have a large strike-slip mechanism. Therefore, we conclude that a large strike-slip fault system did not exist in the western margin of the South China Sea.