Abstract: In western part of Ordos basin Mahuangshan area Mes-ozoic hydrocarbon reservoir has large potential in exploration and development,but how to divide sedimentary facies of Yan'an formation remains a disputable issue.Based on the sedimentary tectonic background,core observation and log-ging analysis,started with sedimentary symbols,built litho-facies and well log facies composition of the study area,the sedimentary facies in yan'an formation has been analyzed.We idengtify Yan5-6 formation mainly belongs to delta plain fa-cies,Yan 7 formation belongs to delta front facies and prodelta facies,and Yan8-10 formation mainly belongs to delta plain facies.In addition,we had worked out Yan5-yan8 formation sedimentary microfacies distribution map in the hope of providing support for better understanding the development situations of reservoirs in the western part of Mahuangshan.