Abstract: The phase transition of pore solution directly affects the physical and mechanical properties of soil during the cooling process. Taking the saline soil of Datong Basin as the research object, the phase transition processes of different saline soils are investigated. The results show that sodium chloride can significantly reduce the freezing temperature of soil. There is a certain deviation between the freezing temperatures of soil and those of free solution, which increase gradually with the increase of salt content. When the concentration exceeds the eutectic point, the freezing temperature of soil remains unchanged, and the deviation of freezing temperature reaches the maximum. The second transition temperature of sodium chloride saline soil is affected by the amount of ice formation. The more the ice crystal content is, the lower the secondary transition temperature is. The sodium sulfate and sodium carbonate have little effect on the freezing temperature. When the concentration is lower than that of the eutectic point, the freezing temperature of saline soil is mainly affected by the salt concentration. If the concentration is higher than that of the eutectic point, the salt crystallization makes the eutectic point temperature of soil further reduce. Under the influence of pore radius, the solubility of salt in pore solution is higher than that in general solution. By comparing the experimental results with the phase diagram, there is a certain similarity between soil and solution. It can be concluded that the secondary phase transition process in saline soil is the formation of salt crystal and ice crystal simultaneously. In the case of high salt content, the freezing temperature of sodium sulfate and sodium carbonate saline soil is actually the eutectic point temperature of the soil.