Abstract: To investigate the mechanism of 2-hydroxyethyltrimethylammonium chloride ( LHDJ) inhibiting hydration swelling of montmorillonite (MMT),the adsorption of LHDJ on dry and hydrated MMT surfaces was studied based on Density Functional Theory (DFT). It is found that LHDJ can be adsorbed stably on the MMT 001 surface and internal surfaces under dry conditions. The adsorption of LHDJ on the Na-001 surface is mainly electrostatic,while the adsorp- tion on None-001 surface and internal surfaces is the interaction of hydrogen bond and electrostatic attraction. The presence of Na ions is not conducive to the adsorption of LHDJ on the MMT 001 surface. In the presence of H2 O,the LHDJ and H2 O exhibit a cooperative adsorption,H2 O bridges the surfaces of MMT and LHDJ and enhances their elec- trostatic interactions and hydrogen bonding. The mechanism of LHDJ inhibiting hydration expansion of MMT:LHDJ produces a large amount of positive ions in aqueous solution,neutralizing the negative charge of the MMT surface and weakening the electrostatic repulsion between the particles. At the same time,LHDJ adsorbs firmly on the surface of MMT,hinders the adsorption of H2 O and weakens the hydration membrane repulsion,meanwhile it does not change the hydrophilicity of MMT due to its short chain length. Furthermore,LHDJ is easy to enter the interlayer exchange of Na ions,and adsorbs firmly with internal surfaces through electrostatic interaction,hydrogen bonding and H2 O bridging. The distance between the layers is shortened,and water molecules cannot easily enter the interlayer to increase the in- terlayer spacing.