The southTianshan orogenic belt, located on the border between the Central Asian Orogenic Belt and the Tarim Craton, is an important area for understanding the tectonic evolution of craton and orogenic belt. However, there are still different views about the timing of closure of the southern Tianshan Ocean and the collision between plates. The Mangqisu pluton is a large composite granite pluton belonging to the eastern part of the South Tianshan Orogenic Belt, and consists mainly of biotite granodiorite unit in the east, biotite granite unit in the west, and two mica granite units in the central part. Our new zircon U- Pb (LA- ICP- MS) results assign emplacement ages of the eastern, western and central units of 297±4 Ma, 294±3 Ma and 292±2 Ma, respectively. Geochemically, samples from eastern and western parts of the pluton show SiO 2 ranging from 65. 34%~73. 12%, A/CNK (1. 00~1. 11, the average is 1. 05), moderate Na 2 O+K 2 O (7. 26%~8. 52%), are of high- K calc- alkaline and weakly peraluminous series. The central granite unit has SiO 2 ranging from 71. 97%~72. 84%, (A/CNK=1. 09~1. 14, with an average of 1. 12), Na 2 O+K 2 O (8. 36%~8. 80%), suggesting that they are of calc- alkaline and strong peraluminous series. They are enriched in LREE ((La/Yb)N=9. 58~44. 94), depleted in HREE (LREE/HREE=8. 00~22. 97), and exhibit negative Eu anomalies, with δ Eu ranging from 0. 67 to 0. 81 in the eastern unit, from 0. 48 to 0. 78 in the western unit, and from 0. 44 to 0. 48 in the central unit. According to their trace element patterns, the studied rocks are enriched in large ion lithophile elements (K, Rb, Ba) and depleted in high field- strength element (Nb, Ta, Ti). The zircon Hf isotopic data of the eastern, western and central units of the Mangqisu pluton show ε Hf ( t ) range from -11. 31 to -1. 64, -5. 41 to -0. 50, and -14. 43 to -2. 51, respectively while the two- stage model age ( t DM2 (Hf)) varies from 1. 30 Ga to 2. 20 Ga. According to the Sr- Nd isotope data, the ε Nd ( t ) value of the eastern section ranges from -6. 92 to -10. 10 , and the two- stage model age ( t DM2 (Nd)) varies from 1. 88 Ga to 1. 62 Ga. Combined with regional geological background and previous research data, we suggest that the Mangqisu pluton can be regarded as an early stage of post- collision granitoids and was derived from partial melting of the thickened metamorphic igneous crust, and experienced different degrees of fractional crystallization.It implies that the closure of the southern Tianshan Ocean and the collision between plates could have occurred before the Permian.