The formation mechanism,crystal structure and application of illite-smectite mixed-layer mineral(I/S) were introduced in detail,based on the previous research.Six kinds of transformation models from smectite to illite were proposed and the advantages and disadvantages of the several models were analysed.The calculation methods of mixed-layer ratio in I/S and their scope of application were detailedly summarized.The I/S's application in the field of geology and research results were described in detail.Finally,two problems await to further research on I/S mixed layer mineral were pointed out.