Combining detrital zircon (DZ) U-Pb age and Lu-Hf isotope data is important in revealing the tectonic evolution of ancient orogens. Here, we conducted DZ U-Pb and Lu-Hf isotope research on 7 modern river sand samples collected from the northern Chinese West Tianshan Orogen (CWTO). The DZs from modern river sands yield ages 340 to 320 Ma and reflect Mesozoic magmatism in the northern Yili Block (YB). In addition, the DZs reflected disproportionate contributions from the source area, which might be affected by the zircon fertility and erodibility of the source rocks. These newly obtained data, together with compiled DZ U-Pb and Lu-Hf isotope data from other modern river sands and ancient sedimentary rocks in the CWTO, are used to constrain the subduction and closure history of the southwestern part of the Paleo-Asian Ocean. The ε_Hf(t) values of 500 to 450 Ma zircons from the southern YB and Central Tianshan Block display a fanning trend (changing from a narrow to a wide range over time), indicating a crustal thickening event at ∼450 Ma. The closure of the North Tianshan Ocean was completed when the YB and an immature/nascent island arc collided in the latest Carboniferous. The South Tianshan Ocean (STO) subducted bidirectionally beneath the Yili–Central Tianshan Block (YCTB) and the Tarim Craton starting at 460 Ma. Slab rollback or retreat of the STO in the northern Tarim Craton starting at 440 Ma caused the opening of several small diachronous back-arc basins. The fanning trend of ε_Hf(t) values indicates that collision between the YCTB and the Tarim Craton occurred at 320 to 310 Ma.