The hydrothermal sedimentary rocks of Lower Cretaceous Bayingebi Formation in Yingejing Depression of Bayingebi Basin has unique sedimentary structure and mineral composition.Methods:Based on the comprehensive study of petrology,mineralogy and geochemistry,this paper describes in detail the characteristics of lacustrine hydrothermal sedimentary mudstone of Bayingebi Formation.Results:The mudstones in the study area can be divided into five types: veined-network mudstones, spotted mudstones,massive mudstones,laminar mudstones and syngenetic deformed mudstones.The main mineral components are dolomite,ankerite,analcite,albite,illite and quartz, and show obvious regularity in the vertical direction.With the increase of depth,the content of dolomite gradually increases,while the content of analcite gradually decreases.The mudstone is rich in Ca,Mg,Mn and other elements,with LREE enrichment,HREE depletion and EU negative anomaly (δCe = 0.96～1.06).Carbon and oxygen isotopes are characterized by positive δ13CvPDB（2.35‰～5.51‰）and negative δ18OvPDB（-1.26‰～-10.16‰).Conclusion:The results show that the hydrothermal sedimentary rocks were formed in a closed saline lake environment with a temperature of 36.23~79.9 ℃ and an average value of 51.21 ℃.The hydrothermal sedimentary rocks belong to the low temperature "white chimney" type.The sulfur isotope has a large variation range (-34.98‰～24.93‰),which show that the pyrite near the vent has a great influence on the hydrothermal solution,and the content of δ34S is relatively high.However,with the decrease of depth,the effect of hydrothermal solution decreased and the effect of microorganism strengthened.The value of δ34S decreased gradually,showing the characteristics of the joint influence of hydrothermal solution and microorganism;Strontium isotopes indicate that the hydrothermal fluid is a mixture of crust source and mantle source.