南海西南次海盆被动陆缘洋陆转换带位于陆缘强烈伸展区，蕴含着岩石圈临界伸展破裂和洋盆扩张过程的丰富信息。本文利用多道地震剖面和重力异常数据，对西南次海盆被动陆缘构造单元进行划分，研究陆缘南、北部洋陆转换带结构构造特征，探讨陆缘伸展演化过程。多道地震剖面资料显示，北部洋陆转换带发育有裂陷期断陷和向海倾斜的掀斜断块；南部发育有低角度正断层控制的裂陷期断陷、海底火山以及局部隆起；从陆到洋方向，重力异常值变化明显。根据上述结果南海西南次海盆被动陆缘划分为近端带、洋陆转换带和洋盆三个构造单元，分别对应了其伸展演化过程的三个阶段：前裂谷阶段、陆缘裂陷阶段和海底扩张阶段。

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.