Abstract: A series of experiments are conducted to analyze the physicochemical properties of the typical weak expansive soil of Anyang before and after treated by ionic soil stabilizer (ISS). Standard moisture absorption content test is used to analyze the water-holding capacity of soil. XRD and FITR tests are employed to identify mineral component and crystal structure, cation exchange capacity and exchangeable cation composition. Zeta potential test is used to analyze the electrochemical properties. The surface area of the expansive soil is also measured. The test results show that the water-holding capacity of the expansive soil decreases after improved by ISS, correspondingly, the swelling capacity weakens. The mineral component of tested soil has no significant change, however the interlayer spacing (d001) shifts to a smaller one, indicating the loss of hydration between interlayer of montmorillonite. Through ion exchange, ISS replaces surface hydrophilic cations in the particles of the expansive soil and promotes a reduction of cation exchange capacity, and the main component of the exchangeable cations is Ca2+. Moreover, Zeta potential and surface area decrease, which enhances the connection between soil particles. The mechanism of action of ISS on the expansive soil can be explained as follows: through a series of complex surface physicochemical reactions including cation exchange, absorption and coverage, the water sensitivity and hydrophilic between interlayers weaken, the structure double electric layer is changed, and the thickness of absorbed water is reduced, consequently, water stability and engineering properties are greatly improved.