The west segment of the Bangong Co-Nujiang structural belt belongs to the east Tethys tectonic domain, including the Bangong Co-Nujiang suture zone, the Gangdise-Nyainqentanglha slab and the South Qiangtang slab. On the basis of stratigraphic correlation for the Mesozoic in the belt and comprehensive study of other geologic features, this paper suggests that the tectonic-sedimentary evolution of the west segment of the Bangong Co-Nujiang structural belt in the Triassic and Jurassic includes the following stages; primary rift (Triassic), proto-oceanic rift (Early Jurassic) and remnant retroarc basin (Middle-Late Jurassic). In the Triassic, a scissors-like half-graben appeared along the Bangong Co-Nujiang suture zone. The primary rift gradually transformed to a proto-oceanic rift in the Early Jurassic, and oceanic crust occurred. In the Middle-Late Jurassic, Neo-Tethys oceanic crust began to underthrust toward the north, extrusion stress stemming from the underthrust gradually made proto-oceanic rift close. Underthrust from the south and north made the Gangdise-Nyainqentanglha slab transform into a magmatic island arc, but elevation differences of the orogenic belt were small. The Oceanic crust of proto-oceanic rift mainly underthrust toward the Gangdise-Nyainqentanglha slab with bi-directional underthrust. In the meantime, regional thermal subsidence occurred, and the proto-oceanic rift transformed into a remnant retroarc basin. The settling center of the basin was near the southern Bangong Co-Nujiang structural belt. Stable continental margin sedimentation which was mainly carbonate developed on the southern Qiangtang slab, and the Jienu Formation which was mainly made up of proximal sediments widely overlapped toward the south in the northern Gangdise-Nyainqentanglha slab. The remnant retroarc basin closed at the end of the Late Jurassic because of further subduction of the Neo-Tethys oceanic crust along the Yarlung Zangbo suture.