Abstract: The Mesozoic stratigraphic distribution in the Ejina area, Yingen-Ejin Basin, has been widely studied.Recent research indicates that the discovered oil and gas in the basin mainly originates from Mesozoic hydrocarbon source rocks.Therefore, the study of the Mesozoic strata in the Yingen-Ejin Basin holds great significance.By analyzing and studying microfossils in the Ejina area of the Yingen Basin, three genera and three species of spores, as well as three genera and six species of protozoans, have been identified.Their coexistence is limited to the Early Cretaceous.Based on this, the previously designated Jurassic was redefined as the Lower Cretaceous Bayingobi Formation, and it was divided into three lithological sections based on rock composition characteristics.Considering the fossil paleoecology, sedimentary structures, and regional geological background, it is believed that the three lithological sections of the Bayingebi Formation represent depositional environments of near-shore subaqueous fans, shore-shallow lakes, and freshwater-brackish inland river-lake on the alluvial plain.This reveals the evolutionary process of the basin from its formation and development to its contraction.The results of the study on the rock composition, sedimentary environment, and fossil assemblages of the Bayingobi Formation contribute to regional stratigraphic research, providing basic data for the study of petroleum geology and regional evolution.