Clay minerals in the outcropped sandstone from Lower Cretaceous Chengzihe Formation and Muling Formation in Jixi Basin were analyzed by X-ray diffraction. The results show that the clayminerals mainly consist of illite, kaollinite and illite / smectite, which can be divided into two types: kaolinite- and illite / smectite types. The outcropped sandstone occurred in middle diagenetic stage-A on the basis of the claymineral composition. The development factor of the formation of kaolinite type clay mineral is caused mainly by the organic acid from the coal-bearing formation and mudstone during the diagenesis process in Lower Cretaceous Chengzihe Formation and Muling Formation in the Jixi Basin. The weak hydrodynamic force of sedimentary facies made the sandstone leaching condition poor, which is the reason forming the aggregation of clay minerals of the illite / smectite- and illite types.