The Sikeshu sag is located in Wusu City,western part of southern margin of Junggar Basin,which contains rich oil,gas and coal resources.The lack of fossil evidence restricts the determination of the geological age of Jurassic strata in Sikeshu sag.Methods: The sedimentary environment and palynology of the Xishanyao Formation are analyzed,the paleoenvironment is reconstructed,and the vegetation succession and environmental evolution during the Middle Xishanyao period are discussed. Results: The lithology of the Xishan yao Formation is mainly argillaceous fine sandstone,light grayish green mudstone,siltstone and mudstone.The Cyathidites minority-Osmundacidite-Cycadopites-Disacciatrileti(COCD)spore assemblage was established by analyzing the sporopollen fossils.The contents of fern spores and gymnosperms pollen in the combination were roughly equal.The fern spores were mainly Osmundacidites of opteraceae,while the podophylla spores also had a certain content.Other common molecules included Deltoidospora and Neoraistrickia,and the gymnosperms were mainly conifer and cypress pollen.All are common species of Jurassic in China.Conclusions: Based on the analysis of the palynological flora of the Xishanyao Formation,it is considered that the palynological assemblage of the Xishanyao Formation belongs to the early Middle Jurassic.The palynological flora of the Xishanyao Formation reflects the characteristics of warm and humid tropical-subtropical climate,and the vegetation type is mainly coniferous and broad-leaved mixed forest.