Abstract: In order to study the mechanism of salt heave and frost heave of sodium sulphate saline soil, the cooling experiments are designed for the sodium sulfate solution at -20℃ environment temperature and in changing temperature ranging from -5℃ to -10℃ and for the sodium sulphate saline soil at -20℃ temperature. It is tried to find the laws of precipitation of mirabilite in the sodium sulfate solution and the sodium sulphate saline soil and their freezing points. The experiments show that at the same environment temperature, a higher concentration of solution leads to an easier precipitation of mirabilite. In the same concentration of solution, a relatively slow environment temperature variation changes precipitation of mirabilite more easily, and the rules of freezing points of the solutions are the same. Compared with that of the sodium sulfate solution, the freezing point of the sodium sulphate saline soil is lower, and at the same environment temperature, salt crystals are more likely to precipitate in the soil.