In this paper, the authors analyzed over 1000 kilometers' multi-channel seismic line (the CFT profile) across the southwestern subbasin of the South China Sea, with identification of seismic interface characteristics and seismic sequence division, and analyzed the detachment effect of detachment faults on the thinning of continental crust in segments. On the basis of the previous research results, an analysis of the crustal structure characteristics of the South China Sea was carried out, and its structural characteristics were summarized. The southwest sub-basin can be divided into four stages of sedimentary response during the series of continental marginal rifting and sea basin spreading and post-expansion: Stage A was the pre-rift stage, showing that the small faults was generated under tensile stress; Stage B was the rifting stage, when the crust of continental margin was thinning in the tensile stress, the fault generated in Stage A was rotated, and the large tilting detachment fault appeared. The sediments were the synrift deposits, and this stage ended with the breakup unconformity; Stage C was the basin expansion stage, during which the sea basin began to expand and the tensile stress shifted from the continental margin to the ocean basin; Stage D was the post-spreading stage, after ending of the basin expansion, the sediments were a set of stable deposits.