Abstract: It is very important to clarify the mechanism of soil-water chemical interaction between acid rain and expansive soil for studying the deterioration mechanism of performance of shallow expansive soil and slope stability in acid rain areas. Therefore, the undisturbed expansive soil in the acid rain-affected areas in Baise City, Guangxi Zhuang Autonomous Region is studied. A cyclic water saturated test device is designed to simulate the rainwater infiltration environment with different pH values to carry out soil-water chemical tests. A variety of micro-test techniques are used to explore the evolution law of minerals and chemical components in the expansive soil. The mechanism of soil-water chemical interaction between expansive soil and acid rain is analyzed by using PHREEQC simulation software and soil-water chemical theory. The results show that compared with the static water saturated environment, the dynamic water saturated environment of rainfall infiltration promotes the dissolution and leaching of minerals in the expansive soil, and accelerates the destruction of soil structure. The dissolution and leaching amount of CaO, Fe₂O₃, K₂O in the expansive soil under acid rain infiltration is significantly greater than that of SiO₂, Al₂O₃. The lower the pH value of rainwater, the more the cements such as calcite and free oxides leached between the sheet microstructures of the expansive soil, and the ion-exchange will be enhanced. The interaction between acid rain and minerals in soil is intensified, and part of illite is converted into montmorillonite clay minerals, resulting in the decrease of illite content, the increase of montmorillonite content, the enhancement of soil hydrophilicity and the increase of structural instability, which is not conducive to the stability of expansive soil slopes.