Abstract: The Permian Wutonggou formation is an important target zone for petroleum exploration in Beisantai swell of Junggar basin, andtypical litho?stratigraphic reservoir has been found in it. Based on the analyses of core and sedimentary facies, combined with cast thin section, SEM, electron micro?probe and X?ray diffraction data, a systematic study of the diagenesis of the Wutonggou formation is presented in this paper. The Wutonggou reservoir underwent compaction, cementation and dissolution, and is in stage between early diagenetic period B and mid?diagenetic period A. The clay minerals in the study area are dominated by authigenic kaolinite and illite/smectite, and the acidic aqueous media and organic matter produce large amount of acid and CO2 during diagenesis, which create favorable conditions for detrital feldspar kaolinization. The rich volcanic debris and low geothermal gradient as well as low speed of smectite illitization are the major reasons for rich illite/smectite in the clay minerals