Abstract: Based on drilling well and 3D seismic data of the Wenchang B Sag in the Pearl River Mouth Basin,the tectonic deformation history and dynamic mechanism of the South Fault were reanalysized. The study shows that the South Fault was characterized by oblique extension during Oligocene and Miocene. On the dip of seismic sections,the South Fault are associated with some specific structural styles,such as squeezed fold and fractural displacement zone. On the strike of seismic sections,the related strata show wavelike lateral extension with thin sediments on "wave peak" and thick sediments in "wave valley". In a plane view,there exist echelon anticlines and depressions,horsetail faults and right diagonal step faults in the Oligocene. The deformation of oblique extension is supposed to be controlled by a regional extension tectonic stress field and the existing early boundary fault. The oblique extension deformation of the South Fault has important effects on the hydrocarbon pools and hydrocarbon phases.