Abstract: Borehole instability is one of the most complicated technical problems in oil and gas exploration and development.The hydration and expansion of clay minerals is the critical factor causing wellbore instability in which the surface hydration is difficult to be removed due to the large hydration potential.In this light, through molecular dynamic simulation, this paper probed into the CaCl₂ inhibitory effect of concentrations, temperatures and pressures on the surface hydration of montmorillonite which revealed the macroscopic mechanism.Results indicated that inhibition of cations were achieved by binding water molecules on the surface of montmorillonite and decreasing the transport and conductivity of water molecules, thereby regulating the invasion of water molecules into the surface of montmorillonite.The ability of inorganic salts to inhibit surface hydration were CaCl₂>NaCl>MgCl₂>KCl.The study found that calcium ions easily adsorbed surface water molecules to form stable outer sphere complex.With the increase of CaCl₂ concentration, coordination number, hydration number and hydration radius of calcium ion decreased, and the inhibitory effect diminished.When the temperature increased and the pressure decreased, the conductivity and transport capacity of water molecules was enhanced in the system, the coordination number of calcium ion descended, and mechanical strength declined.