Abstract：The Tengger Mountain-Ewirgol area is located in the boundary region of the East and West Tianshan Mountains. For a long time，no unified classification, age assignment, distribution range or nomenclature of stratigraphic units have brought many problems to the study of the Tianshan orogenic belt. Through 1∶50000 regional geological surveys of six sheets of the East and West Tianshan Mountains, two unconformities and a wealth of fossils have been found in the area of the original Qiergustao (Tengger) Formation in the North Tianshan Mountains. On that basis, combined with the differences in rock association, metamorphism and deformation, depositional setting, and isotope age data, the original Qiergustao (Tengger) Formation is further disintegrated into the Upper Devonian Tengger Formation (senso stricto), Upper Carboniferous Qiergustao Formation (senso stricto) and Houxia Formation and Lower Permian Aerbasayi Formation, and their ranges of distribution are mapped and the tectonic framework of the area is redefined. This study provides an important scientific basis for the correlation of stratigraphic units and tectonic division in the boundary region of the East and West Tianshan Mountains.