Abstract: Based on seismic interpretation data, this paper performs an analysis on tectonic evolution and formation of fault zones in Qikou sag of Bohai Bay Basin, and divides tectonic evolution of Qikou sag into three stages: rifting episode I, rifting episode II and post-rifting episode. In the rifting episode I, Qikou sag showed a rift structure. The preexisting NNE- and NWW-trending faults extended obliquely, and the NE-trending faults were newborn faults. In the rifting episode II, Qikou sag had a fault-depression structure, and the preexisting NNE-, NWW- and NE-trending faults were reactivated, and the EW-trending faults were newborn faults. In the post-rifting episode, Qikou sag showed a depression structure, and different types of fault zones were formed by multiple phases of superimposed oblique extension and deformation. The fault zone can be divided into five types: the first type, i.e., multi-phase extending fault zone, is formed by the preexisting pre-Paleogene NNE-trending extensional faults superimposed with the Paleogene extensional deformation; the second type, i.e., transfer-extension fault zone, is formed by the preexisting pre-Paleogene NWW-trending transfer fault superimposed with the Paleogene extensional deformation; the third type, i.e., single-stage extensional fault zone, is formed by the Paleogene extensional fault; the fourth type, i.e., extension-transfer fault zone, is formed by the preexisting pre-Paleogene NNE-trending extensional fault superimposed with the Paleogene transfer deformation; the fifth type, i.e., transverse anticlinal fault zone, is developed under the control of the Neogene transverse anticlines. At each evolutionary stage, the formation, scale, and migration characteristics of subsags in Qikou sag were controlled by segmental growths of the main fault and its angle with the regional extension direction. The tectonic evolution of Qikou sag has an important influence on the development of source rocks, trap types, and hydrocarbon migration.