The Yarlung Zangbo Suture Zone (YZSZ) on the southern margin of the Lhasa block and the Tangjia-Sumdo ultrahigh-pressure metamorphic belt (TSMB) within the block represent natural laboratories for the study of plate tectonics and oceanic slab subduction. It is generally believed that these two zones represent the remnants of the ancient oceanic crust and upper mantle of the Yarlung Zangbo Neo-Tethys (YZNT) and the Tangjia-Sumdo Paleo-Tethys (TSPT). However, the evolutionary patterns and spatiotemporal relationships of the TSPT and the YZNT have been debated. In this paper, the spatiotemporal evolutionary sequences of the TSPT and YZNT (eastern segment) are systematically summarized based on the previous data and research results of our team, and the relationship between the two is discussed in depth. We propose the following: (1) The TSPT and YZNT represent different evolutionary stages of one ocean. (2) The complex material between the TSMB and the YZSZ represents the geological record of the continuous northward subduction and southward recessional accretion of the TSPT since the Late Paleozoic, which we define as the Southern Gangdese Accretionary Belt. (3) The Southern Gangdese Accretionary Belt consists of Tangjia-Sumdo accretionary complex (P-T₂), Qushui-Lilong accretionary complex (T₃-J₁), and Gaxue accretionary complex (K_1-2) from north to south, and the sedimentary rocks in the Mailonggang-Xionglai Basin (T₂-J₁) and Lhasa-Riduo Basin (J₂-K₂) unconformably overlie the accretionary complex (wedge-top basin), representing a compound accretionary basin–mountain system.