Abstract: The thickness of adsorbed water layer plays an important role in the plasticity, absorption, permeability, swelling, conductivity, strength, deformation and physical-mechanical properties of red clay particles. In order to analyze and calculate the thickness of adsorbed water of red clay before and after treatment with ionic soil stabilizer (ISS), the Atterberg test, Zeta potential test, conductivity and specific surface area test are carried out. According to the model of flat clay particles, a formula for the thickness of red clay particles is deduced. In terms of liquid limit, specific surface area and density of adsorbed water, the thickness of adsorbed water is calculated. The results show that ISS can reduce the plasticity index of the red clay, Zeta potential and conductivity so as to decrease the thickness of adsorbed water, but the structure of electric double layer can not be changed and the adsorbed water can not be removed completely. The experimental and theoretical calculations show that the reduction of plasticity index to determine ISS reducing adsorbed water of red clay is feasible and effective.