Abstract: As a typical half-graben in the deep water area of the southern Qiongdongnan basin, the Beijiao sag with the "leg bend-like" deformation of its sequence stratigraphy is consistent with the model of extensional fault-bend fold. Using the theory of extensional fault bend fold and balanced cross-section, we reconstructed the evolutionary process of the Beijiao sag and Beijiao low-uplift in terms of extentional tectonics. After analysing and measuring the structural deformation of the sag, the geometric character and kinematic process of the fault bend fold are established. It is estimated that the sag totally extended 12 km with an average extension rate of 0.44 km/Ma during the period from 50 Ma to 23 Ma, and the sea level of this area rose about 200 m by the end of Eocene.