Abstract: Southern South China Sea, where abundant geological information and hydrocarbon preserved, is the key to figure out the tectonic evolution of Southeast Asia. The southern South China Sea has been attracting attentions because of that. However, tectonic framework of southern South China Sea is complex, which hinders the researches on it. The Beikang Basin locates in the southern South China Sea. The basement−involved structures in the Beikang Basin are relatively simple, which can be a key to the evolution of the southern South China Sea. Seismic interpretation, qualitative and quantitative analysis of faults and the extensional factor analysis of lithosphere are used to analyze the structural characteristics of the basin. Analysis suggests that the prototype of the lower Beikang Basin is rift basin, locally detachment basin, bounded by the Saba Orogeny unconformity. The prototype basin of this section is rifted basin. The basin locates in the thinning zone of the passive continental margin. The formation of the rift basin is mainly related to the pull of the subducted proto−South China Sea. The entirely different evolutional histories of the Zengmu Basin and Beikang Basin testify the existent of the West Baram Line and suggest new division of geodynamic provinces for the South China Sea. The area was divided by the West Baram Line into collision−extrusion basin group and proto−South China Sea slab pull basin group. The strike−slip of the line began at Late Eocene and ended at latest Early Miocene. Comparisons between basement−involved structures in the southern and northern South China Sea reveal that deformation concentration towards the oceanic crust can only be found in necking zone and distal zone of the South China Sea margin, while deformations away from the ocean are rarely affected by seafloor spreading. The variety of extensional tectonics perpendicular to the passive margin may be associated with the decoupling between the proximal and distal margin before seafloor spreading.