Abstract: It is quite controversial whether there is a regional angular unconformity between the Upper and Lower Carboniferous strata in Western Tianshan Mountains, and such a situation restricts the division of the Late Paleozoic tectonic evolution. The Carboniferous strata are typical and intact in Yili. In this paper, the sedimentary and petrographic studies were carried out by redefining the contact relationship between the Upper and Lower Carboniferous strata in this area. The sedimentary characteristics show that these two strata are consistent, without conglomerate and weathering crust, and are characterized by mixed carbonate and volcanic debris. The Lower Carboniferous Akeshake Formation and Upper Carboniferous Yishijilike Formation may have no significant depositional interruption, and should be a continuous volcanic-sedimentary sequence. This study provides sedimentary evidence for the understanding of the Carboniferous as a unified post-collision rift basin in the Yili area, Western Tianshan Mountains.