Abstract: Sulphate saline soil is widely distributed in the northwest of China.Salt expansion is harmful to the highways in the saline soil area.Studies on the laws of salt expansion were of great significance for harnessing the salt expansion diseases.Considering that the temperature interval of the deformation of sulphate saline soil subgrade was divided into three stages,an equation showing the relationship among salt expansion ratio ?,initial moisture content θ1,initial voids ratio n and solubility r1 and r2 in the course of the pure salt expansion of the sulphate saline soil was established on the basis of a series of hypotheses.The rationality of the salt expansion equation was confirmed by the test system of soil under controlled temperature.