Abstract: The origin of the South China Sea (SCS) still remains a dispute. With emphasis on internal mechanisms for the formation and evolution of the SCS, based on geological interpretation of a large number of seismic profiles, and using tectonic analysis as the method, the authors carried out systematic tectonic restoration of the structural deformation features of the Pre Cenozoic basements on the northern and southern continental margins of SCS. Back thrust fold structures are found remained in the basements, which are endemic to collision orogenic belt. The axis of the paleo orogenic belt is distributed roughly along the modern Shuangfeng Bijia seamount line. We name it "Paleo Shuangfeng Bijia collision orogenic belt" hereafter in this paper. Together with the analyses of Cenozoic magmatite geochemical characteristics of SCS region and the Cenozoic extensional tectonic characteristics of northern and southern continental margins, the authors proposed that the orogenic belt underwent a mountain root delamination process in Cenozoic. And the mechanism of "mountain root delamination" is the fundamental origin of the SCS.