Abstract: Based on the characteristics of electrochemistry, a new idea for in-situ treatment of expansive soils is proposed, with a high charge hydroxy aluminum ion solution as the electrolyte, and the modification of montmorillonite by hydroxyl aluminum ions into the soils under the effect of applied electric field. From the macroscopic point of view, the particle analysis is conducted, and the limiting moisture content and swelling characteristics of montmorillonite before and after electrochemical modification are studied. The results show that the swelling characteristics of the modified montmorillonite decrease significantly, and the soils in the anode area reach the performance requirements of the non-expansive soils. The X-ray diffraction tests on the montmorillonite before and after modification are carried out from the microscopic layer. The hygroscopic sensitivity between the lattice layers of the montmorillonite after modification is obviously reduced, indicating that the hydroxyl aluminum ions enter into the interlayer, replace the cations and reduce the hydrophilicity of the mineral lattice. The changes in the weight loss of montmorillonite before and after the modification at different temperatures are studied through thermal analysis. It is shown that the hydroxyl aluminum ions enter into the double layers to exchange cations and attach to the surface of clay particles, reducing the thickness of the double layers and the expansibility of the montmorillonite.