Abstract: During cooling process, solubility of sodium sulfate decreases, and part of sodium sulfate in the solution precipitates and crystallizes. When the temperature is lower than the specific temperature, in addition to crystallization of sodium sulfate, liquid water also precipitates as ice. The crystallization of the aqueous solution in porous medium induces the deformation and failure of porous medium, such as saline expansion and frost heave deformation of saline soils with sulfate sodium, salt efflorescence of wall painting, degradation of stone and concrete, etc. To study the deformation and failure mechanism of porous medium with saline solution during cooling process, the method for calculating the volumetric change rate of the system of Na₂SO₄+H₂O during cooling process is proposed. In the procedure of calculation, the content of phases is determined according to the equilibrium concentration of phases in the system, Na₂SO4+H₂O, and the volume of the phases is added to calculate volumetric change rate of the system. Compared with the existing tests, the proposed method can successfully predict the volumetric change rate. The saline expansion rate is smaller than frost heave rate during cooling process. In addition, when heptahydrate sodium sulfate crystallizes and precipitates during at the saline expansion stage, the volumetric change ratio is relatively small, but the volumetric change abruptly increases at the frost heave stage.