The improvement of expansive soil with chemical medicine has been concerned by geotechnical engineers. The cation modifier is used to improve the expansive soil in embankment engineering of an express highway in Yunnan province. The modified test results indicate that the hydrophilic nature and swelling-shrinkage properties of samples are changed drastically. The free swelling ratio decrease and the particle size gradation, strength characteristics and hydrological properties of the treated expansive soil are improved by a wide range. The clay content decreases, but the silt content increases. The void ratio decreases sharply, but the fabric becomes more compact. The change of dry density with water content changing is less, but the water stability of modified soil is improved. The indexes are satisfied with the project requests and the engineering cost reduces.