The invention discloses a method for modifying expansive soil by waste foundry sand, which comprises the following steps: step one, according to the proportion of between 1 to 5 and 1 to 3 of the mass of the waste foundry sand to the mass of dry soil of a soil body needing to be improved, weighing the waste foundry sand and the soil body to be improved, and mixing the waste foundry sand and the soil body evenly, wherein the components of the waste foundry sand in mass percentage are: 41 percent of quartz sand, 37 percent of coal dust, 19 percent of clay and 3 percent of iron; step two, determining the optimum moisture content; and step three, adding water to the mixed soil obtained in the step one, and stirring the mixture evenly to obtain waste foundry sand improved expansive soil. The method for using the waste foundry sand to improve the expansive soil is utilized to obtain a construction method for using the waste foundry sand to improve the expansive soil. With a method for constructing the waste foundry sand improved expansive soil which is obtained by the method for improving the expansive soil by the waste foundry sand, the obtained waste foundry sand improved expansive soil is paved as a surface improved layer the thickness of which is between 30 and 60 centimeters on the construction surface; and then the surface improved layer is kept for 3 to 7 days under natural condition, and the improved soil body has a chemical reaction with the components of the waste foundry sand to eliminate or reduce the swell-shrink characteristics of the soil body in the improved layer, thus the strength required by an engineering can be achieved.