The improvement of expansive soil is an important lesson in the study of expansive soil subgrade treatment; so it has been concerned by the geotechnical researchers and engineers. Combined with slope treatment project of expansive soil on a highway

the chemical improving expansive soil by experiment with ESR ecotypic medicine is studied. Both of the mechanism and the effectiveness of improving expansive soil are analysed. It is discovered that the expansive soil become non-expansive after chemical treatment. The improved expansive soil has been changed from hydrophilicity to hydrophobicity and has good water stability. While the soil has the characteristic of sand and every strength index is largely increased. The foundation bearing capacity is also increased by the number of spraying; and the best spraying number is 3－4.