Abstract: The improvement of expansive soil with chemical medicine is an important topic in chemical-geotechnical engineering,so it has been concerned by geotechnical engineers.For the embankment engineering on an express highway in Yunnan Province,the expansive soil is improved with the cation modifier.The mechanism and the effectiveness of the improved expansive soil are discussed.It is discovered that hydrophilic nature and swelling-shrinkage property are changed drastically,that the free swelling ratio can be reduced,and that the particle size gradation,strength characteristics and hydrated properties of the treated expansive soil are improved by a wide range.Clay content decreases,but silt content increases.Pore decreases sharply,but the fabric is compact.The effectiveness of improving medicine reunited and cemented is obviously.The change of dry density with water content decreases and the hydrated property of modified soil is enhanced.The indexes are satisfied to the project and the cost is reduced.So the engineering properties of the treated expansive soil are improved to a considerable extent.