Abstract: Based on the data interpretation on two long seismic transections across the southern and northern continental margins of the South China Sea, we analyzed the activity characteristics of the main sag-controlling faults in the South China Sea and the initial tensile fracture characteristics of the Cenozoic crust. Most of the faults began to develop and move strongly during the Cenozoic period of tensile fracture. The sag-controlling faults are usually developed at the sag boundary, also known as the sag-controlling boundary faults. In the early stage, the most sag-controlling faults are characteristic of rotation and detachment of fault blocks and the formation of listric faults. Most of the main sag-controlling faults in the northern continental margin are episodic faults in characteristics, and the initial activity time is mainly from the Early Eocene to Late Oligocene. In particular, the initial activity time of faults in the eastern part is the Early Eocene or earlier, and that in the western part is the Middle Eocene or later. In the southern continental margin of the South China Sea, the initial activity time of sag-controlling faults is later than that in the northern continental margin, mainly concentrated in the Eocene or Late Eocene. It can be seen that the initial activity time of the Cenozoic crustal tension in the South China Sea presents the characteristics of "early in the east and late in the west, and early in the north and late in the south". In addition, some of the sag-controlling faults continued or began to move during the drifting period of the South China Sea, and a few faults reactivated in the Miocene.