Abstract: The age and subdivision of the Triassic Karamay formation in the northwestern margin of the Junggar basin are quite different in references, which have negative impact on the related geological researches and thepetroleum production. Based on reserches on Trassic biostratigraphy and fossils collected by present work, this paper makes comprehensive studies on the biostratigraphy of surface and subsurface. By the discovery of Late Triassic Danaeopsis fecunda from the S7 sand unit of the lower member of the Karamay formation, this paper proposes that the age of the upper member of the Karamay formation in the northwestern margin of the Junggar basin should be Late Triassic, while that of the lower member of it should be late Middle Triassic to early Late Triassic; the boundary between the Middle Triassic and the Upper Triassic is located in the S7 sand unit. The Karamay formation overlies unconformably the underlying Baikouquan formation,and the deposits of late Early Triassic and early?middle Middle Triassic. The piedmont of the Zaire mountain appeared to be an eroded area in the Early?Middle Triassic and receive depositsin the Late Triassic, experiencing the evolution of the sedimentary facies from alluvial fan,braided river to lacustrine sedimentary systems. In the middle?late Late Triassic, the sedimentary stage of the Baijiantan formation was deposited in open lacustrine basin