The petrology and origin of kaolinite rocks under coal No.9 of Taiyuan Formation in Datong coalfield were studied based on the polarized microscope. Four texture components, i.e. clast (including terrigenous clast,volcanic quartz crystal pyroclast,volcanic biotite crystal pyroclast,devitrified"flint-like" vitric clast), cryptocrystalline kaolinite graupe, kaolinite crystal clast and kaolinite matrix, were identified under polarized microscope. The research shows that the main rock types of this bed of kaolinite rock were tuffaceous graupe kaolinite rock with thin feldspar pseudomorphic crystal kaolinite rock on the top, and intercalated with tuffaceous sandstone bed at middle part. This bed of kaolinite rock was originated from the tuffaceous sediments reworked by water current at the basin bottom. Especially, the new formation mechanism of abundant graupe were firstly submitted:that is, the cryptocrystalline kaolinite graupe is the altered products of silica-alumna vitric clast, which was reworked by water current in the basin bottom and show no evidence of volcanic origin.