Abstract: The swelling characteristics of interlayer spacing (d₀₀₁) of Na-montmorillonite (Na-MMT) are analyzed in different polymer and electrolyte solutions. It is found that due to its huge volume in aqueous solution, the polymer molecule can not inserted into the interlayer of Na-MMT and hardly has remarkable effect on the expansion of the interlayer spacing; on the other hand, the electrolytes, especially KCl, can inhibit the expansion of the interlayer spacing. In addition, the inhibition ability varies with the category and concentration of the cations. Further studies show that the process of interlayer spacing expansion is accompanied with the ion-exchange between the MMT and the solution, and that the ion-exchange capacity exhibits certain correlation with the change of interlayer spacing. The interaction between the cations and the internal space of the crystal layer is the key factor to influence the expansion of interlayer spacing, and interlayer spacing is finally determined based on the concentration, ion hydration radius and the ability to attract crystal layer of the cationic.