The function of clay minerals and the effect of clay mineral dehydration on land subsidence have become an increasingly important key point in the evaluation and solution of many problems in such aspects as geology,engineering geology,hydrogeology,environmental geology,and hence aroused more and more attention in geologists. This paper describes characteristics of montmorillonite interlayer water and factors affecting the swelling with hydration and the contraction with dehydration. The solid solution properties of the montmorillonite interlayer are pointed out. Based on the achievements made in investigating the relationship between montmorillonite dehydration and land subsidence,this paper puts forward the train of thought for further study of the secondary land subsidence in the light of montmorillonite dehydration. The possibilities of using the montmorillonite dehydration to the study of delayed land subsidence and the prediction of land subsidence are also discussed in this paper.