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The unfrozen water content is a result of soil freezing, and its variation is closely related to temperature. To study the change in unfrozen water content, a relationship between water pressure and ice pressure is built based on the Clapeyron equation. Then, using an assumption of coincident faces between the water–ice interface and the water–air interface, a formula is deduced to estimate the unfrozen water content via the soil-water potential curve combined with the capillary pressure theory. In this process, the soil particle size distribution curve is introduced to determine the soil pore-size distribution and obtain the soil-water potential curve. Finally, verification and analysis are performed by comparing these results with the pre-existing test data for unfrozen water content. The results show that unfrozen water content can be calculated by means of the soil particle size distribution curve, and the simulation accuracy has a positive correlation with the degree of saturation occupied by water. Moreover, the equivalent aperture size increases as the mean grain size of the soil increases, and the equivalent aperture size increases sharply and approaches the mean particle size when the mean is within a range of relatively large values. Additionally, the value of scale parameter α has an obvious influence on the unfrozen water content curve, as enlargement of α causes an increase in the unfrozen water content and makes water freezing in soils more difficult, and vice versa. [ABSTRACT FROM AUTHOR]