Abstract: Based on the seismic profiles through the Qiongdongnan Basin, the authors found that the No.12 fault in the Southern depression is a transfer fault, after analysis of the spatial extension and 2-D section of the fault as well as the associated stratigraphic framework. The No.12 fault is the accommodation structure between the Huaguangjiao Sag and Yuzhuojiao Sag. The fault has experienced three stages of evolution. It began with the rift extension from Eocene to Oligocene. In this stage, the No.12 fault made an intense move under the control of the NE-NNE trend fault between the Huanguangjiao Sag and the Yuzhuojiao Sag, such as Huaguang No.2 fault and No.11 fault. In the post-rifting subsidence stage of Miocene, the No.12 fault stopped movement due to the inactivity of the main faults of the sags in southern depression. Finally, in Late Miocene, the No.12 fault remained active as a strike-slip fault resulting from the convergence of the Luzon Arc westward.