The northern margin of Alxa in Inner Mongolia is located in the southern margin of the middle part of the Central Asia orogenic belt, It is an important area to study the spatia and temporal evolution of Central Asia orogenic belt, and it is also a key position of the connection between Beishan orogenic belt and Xingmeng orogenic belt. Because of the bad climate and inconvenient transportation in this area, the basic geological research is relatively low, and the Late Paleozoic stratigraphic units are chaotic. In this paper, based on the study of the newly discovered brachiopod and coral fossil age in southwestern Yagan, and combined with the detrital zircon latest peak age 281.9±3.4Ma obtained from the lithic sandstone in the south of the fossil point, a set of shallow-seashore facies clastic rock assemblage classified by the predecessors as the “Upper Permian” is reclassified as the Shuangbaotang Formation, and the age is placed in the late Early Permian to the Middle Permian. Through the study of rock association, stratigraphic sequence and fossils, the geological evolution history of Early-Middle Permian in Yagan area was restored.