The time frame for the collision between Siberia Plate and Kazakhstan-Junggar Plate and the initiation of post-collision of the Southern Altay Range in the Late Paleozoic remain controversial. Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Southern Altay Range and various origins of granitoids have been found in this region, including I-, S-, A- and M-type granitoids. Here, we report the newly discovered Duomolakebulake garnet- and muscovite- bearing leucogranite which intruded the Early Carboniferous Nanmingshui volcanic-sedimentary formation in the southern Altay Range. LA-ICP-MS zircon U-Pb dating indicates that the leucogranite was generated in the early Late Carboniferous (311+3 Ma). As an important rock type for tectonic implications, leucogranite is considered to be generated in a post-collision setting. In combination with lithology data in this region, we suggest that the collision ended prior to ~311 Ma in the southern Altay Range, and then evolved into the post-collisional stage. This new discovery may shed lights on the tectonic evolution of the Altay Range in the Late Paleozoic.