Abstract: Kaolinite is one of the main clay mineral components of coal measures mudstone,and its existence has an important influence on the hydraulic action of mudstone.The molecular mechanism of the interaction between kaolinite and water molecules is the basis for an in-depth understanding of the phenomena of mud-stone expansion,softening,disintegration and even mudding.Molecular dynamics simulation technology is a powerful tool to reveal the relationship between material structure and properties and to understand the mechanism of matter interaction in physical and chemical systems.The water absorption characteristics of kaolinite were simulated by grand canonical ensemble Monte Carlo (GCMC) and molecular dynamics (MD) methods.The effects of temperature and pressure on the adsorption capacity,adsorption site,adsorption heat,adsorption energy and volume expansion of kaolinite particles were studied,and the molecular mechanism of water absorption of kaolinite was clarified.The results show that in the process of water molecules adsorption by kaolinite,water molecules and surface atoms of kaolinite form hydrogen bonds in the form of Hw-Ok and Ow-Hk.The adsorption heat is between 1.898 kJ/mol and 2.133 kJ/mol,which is far below the critical value of chemisorptions 42 kJ/mol,and the interaction between kaolinite and water molecules is a typical physical adsorption.The adsorption energy is negative,the thermodynamics of kaolinite system is stable after water absorption,and the changes of adsorption energy value,adsorption capacity and volume expansion are similar,which increase logarithmically with the increase of pressure,and decrease with the increase of temperature and the decrease is relatively small.The adsorption site is the frequency of water molecules appearing at a certain position on the surface of kaolinite particles during the adsorption process.The adsorption of water molecules on the surface of kaolinite particles depends on the van der Waals energy and hydrogen bonding energy between molecules,and the effect of pressure on the adsorption characteristics is greater than that of temperature,and high temperature is not conducive to the adsorption of water molecules on the surface of kaolinite particles.