The present paper addresses two tectonostratigraphic concerns on the Late Paleozoic Tianshan tectonic complex (TTC), Xinjiang, Northwest (NW) China: (1) stratigraphic succession and age constraint of the Bayingou ophiolite mélange, eastern Tianshan Mountains and (2) timing of closure of the southern Tianshan ocean and accretion of the Siberian craton recorded in the Aiweiergou (AWEG) area, eastern Tianshan Mountains by integrating stratigraphy, palaeontology, tectonopalaeogeography and palaeobiogeography. In the Bayingou area, the detailed palaeontological survey denies the presence of brachiopod Gigantoproductus fauna, typical of the Early Carboniferous faunas in central-south Tianshan complex, in the Anjihai Formation. In contrast, the Anjihai brachiopod assemblage, as a whole, appears to have a high affinity with the Late Devonian faunas of the eastern Junggar Basin, northern Xinjiang, suggesting a Late Devonian age for the Anjihai Formation. The overlying Shadawang Formation yields the Early Carboniferous radiolarians. These two units form the main part of the Bayingou ophiolite mélange, which therefore is likely Late Devonian to Early Carboniferous in age. The Bayingou area has been likely part of the northern Tianshan-Junggar block since the Late Devonian, although it may have been part of the Central Tianshan tectonostratigraphic province prior to the Late Devonian. The topmost strata of the Bayingou ophiolite mélange are characterized by alternation of volcanics, conglomerate and mudstone, and are better re-assigned to the Taoxigou Group rather than the Keguqingshan Formation. The Bayingou ophiolite mélange comprises the Late Devonian Anjihai Formation, the Carboniferous Bayingou and Shadawang Formations, and the Early Permian Taoxigou Group. In the AWEG area, the Permian and Triassic rocks were previously misinterpreted as the Late Permian turbidites and Late Triassic red beds, respectively. In fact, the Permian successions in AWEG consist of the Early Permian Taoxigou Group and early Middle Permian Lucaogou Formation. The former represents a foreland molasse succession, while the latter yields abundant non-marine fossils of plants, bivalves, and gastropods, and represents typical lacustrine facies deposits. The unconformity between the Permian and the Triassic rocks cannot represent the closure of the Tianshan Ocean, but indicates tectonic uplifts in the foreland basins. In contrast, the molasse-type sediments of the Lower Permian Taoxigou Group may have resulted from the post-orogenesis uplifting and mark the closure of the Tianshan Ocean prior to the Early Permian. Thus, the closure of the Tianshan Ocean and the final tectonic accretion of the South Tianshan block might have taken place over the Permo-Carboniferous transition, strengthened by faunal assemblages obtained from both southern and northern sides of the TTC.