The passive continental margin ocean-continent transition zone of the southwest subbasin of the South China Sea is located in a strong extension area of the continental margin, which contains rich information about the critical extension and rupture of the lithosphere and the expansion of the ocean basin. In this research, the multi-channel seismic profile and gravity anomaly data were used to classify the passive continental margin structural units of the southwestern subbasin, the structural features of the southern and northern ocean-continent transition zones of the continental margin were studied, and the evolutionary process of continental margin extension was discussed. Multi-channel seismic profile data show that the northern ocean-continent transition zone develops rift-period rifts and tilting fault blocks that tilt toward the sea; in the southern, there are rift-period rifts controlled by low-angle normal faults, submarine volcanoes, and local uplifts. From land to ocean, the gravity anomaly changes significantly. According to above results, the passive continental margin of the southwest subbasin of the South China Sea is divided into three structural units: the proximal zone, the ocean-continent transition zone and the ocean basin, corresponding to the three stages of its extensional evolution process: pre-rift stage, continental margin rift stage and seafloor spreading stage.