Abstract: The Bayan Har Basin, situated in the northeastern Tibetan Plateau, hosts the Triassic Bayan Har Mountain Group, a critical product of the Mesozoic tectonic evolution of the Tethyan Ocean. This unit provides significant insights into the evolution of the Songpan–Ganzi terrane and the Tethyan tectonic domain. However, the provenance of the Bayan Har Mountain Group remains highly debated. In this study, three stratigraphic sections from different units in the northern Bayan Har region were investigated: the Changmahe Formation (T_1-2c), likely deposited in a continental slope setting, exhibits detrital zircon U-Pb age peaks at 309 Ma, 248 Ma, and 438 Ma; the Gande Formation (T₂gd), interpreted as a relatively stable deep-sea submarine fan, shows dominant age peaks at 431 Ma and 255 Ma; and the Qingshuihe Formation (T₃q), inferred to represent a platform margin-shallow marine shelf environment, displays age peaks at 276 Ma, 433 Ma, and 1 896 Ma. Provenance analysis reveals that the Middle-Lower Triassic strata were primarily sourced from the A’nyemaqen tectonic belt to the north, while the Upper Triassic strata incorporated additional contributions from the exhumed Kunlun Block. This shift in provenance signals the probable closure of the A’nyemaqen Ocean by the late Middle Triassic, followed by Late Triassic uplift of the Kunlun Block, which became a major sediment source for the northern Bayan Har Basin. These findings constrain the timing of paleogeographic reorganization and enhance understanding of the Tethyan tectonic evolution in the eastern Tibetan Plateau.