The Zhongjiannan basin is located in the west continental margin of the South China Sea (SCS) and belongs to a strike- slip extensional basin. The basin has the dual structural characteristics of tension fracture in the northern SCS and strike slip in the western SCS. Several regional unconformities were developed in the basin inthe Cenozoic, which is of great significance to the study of sedimentary- tectonic evolution history, oil and gas resources and the formation process of the SCS. In this paper, we re- interpreted the deep reflection seismic profiles of Guangzhou Marine Geological Survey by collecting the drilling data of Vietnam, and recalibrated the geological attributes of two regional unconformity surfaces T5 and T3 in the post- expansion period of the SCS. Combined with the analysis of regional tectonic movement, it is proposed that the surface T5 belongs to collision unconformity surface, and mainly formed in the period of the seafloor spreading stop. This surface corresponded to the transition period from left- lateral movement to the right- lateral movement of the Red River- Western edge fault zone withan age of 15 Ma. The seismic profiles reveal that the overall tectonic activity was not strong in T5 period, reflecting that the response of Sabah orogeny in the Zhongjiannan basin had been greatly weakened. The unconformity T3 is a tectonic transition surface, which reflects regionally important plate reorganization event and a rapid decline of global sea level. This surface is closely related to the right- lateral strike- slip of Red River- Western edge fault zone, with an age of 11. 6 Ma. Seismic profiles show this interface has obvious denudation and truncation characteristics, which were the result of strong action of the Nansha Movement or the Wanan Movement in the basin during the important period when large- scale carbonate rocks, channels and fans began to develop in the western SCS. Our research indicates that there are significant differences in tectonic activity and sedimentary response during the post- spreading period of the SCS. The spatial variation characteristics and regional dynamic environment of the surfaces T5 and T3 in the western SCS can be clarified by ocean drilling in the future, which will provide a basis for understanding the controlling effect of Red River- Western edge fault zone of the SCS on the genetic mechanism of the western sedimentary basins and the difference of oil and gas distribution.