The dry-wet cycling test, the free expansion rate test, unconfined compressive strength test are carried out for the expansive soil, lime improving expansive soil, the steel slag fine powder modified expansive soil to compare and verify the modified effect.The results show that the steel slag fine powder has improved significantly effect on the action of the expansive soil dry-wet circulation development of crack, the reduction of free expansion rate, the increase of strength.Among them, the modified effect of steel slag fine powder to free expansion rate is obviously behind the lime, but the ultimate improvement effect is similar.Strength improvement of steel slag fine powder is superior to lime. With the method of X-ray energy spectrum, the mineral composition and chemical composition of the expansive soil, lime improving expansive soil and the steel slag fine powder modified expansive soil are analyzed.The conclusion is that the mechanism of modifying expansive soil by steel slag fine powder is similar to the lime.