【Objective】Metastable equilibrium is an important theoretical paradigm to describe the characteristics and extent of the actual reaction equilibrium in ion-exchange adsorption. This study aimed to theoretically derive the metastable equilibrium model of ion adsorption, establish the mathematical relationship between ion equilibrium adsorption amount and activation energy, and discuss the specific effect of alkali metal ion metastable equilibrium adsorption. 【Method】Using miscible displacement technology combined with the ion adsorption kinetic model and based on surface electrostatic field theory, the metastable equilibrium adsorption of K⁺, Na⁺, and Li+ on the surface of saturated Cs⁺-montmorillonite was investigated. The theoretical model was verified by carrying out adsorption kinetics experiments of K⁺, Na⁺, and Li⁺ on the Cs⁺-montmorillonite surface. 【Result】(1) Significant metastable equilibrium characteristics were observed during K⁺, Na⁺, and Li⁺ adsorption onto the surface of Cs⁺-montmorillonite saturated samples, and the adsorption activation energy (energy barrier) of exchange ions came from the desorption process of adsorbed Cs⁺; (2) Strong ion polarization caused by the surface electrostatic field resulted in specific effect of metastable equilibrium adsorption of each ion under the same ion concentration condition; (3) The metastable equilibrium of ion adsorption was affected by both ion concentration and specific ion effects, and the ion equilibrium adsorption capacity was proportional to the ion adsorption rate. 【Conclusion】The adsorption equilibrium of alkali metal ion at the montmorillonite surface is a metastable equilibrium rather than a real equilibrium. The presence of ionic metastable adsorption equilibrium should be attributed to the restraint of activation energies to ion adsorption kinetics. Also, the different polarization effects of alkali metal ions lead to different activation energies of each system, which further triggers the specific effect of ion metastable equilibrium adsorption.