<p>Soil freezing and thawing are critical for understanding water–heat dynamics in cold regions. The initial freezing point and unfrozen water content (UWC) are key parameters for describing soil freezing and thawing characteristic curves (SFCC/STCC), which capture differences between the two processes. Accurate simulation of these curves, along with clarification of the hysteresis mechanism of UWC, is essential for improving model performance. In this study, we establish the relationship between contact angle and supercooling temperature based on heterogeneous nucleation theory. The initial contact angle is inversely estimated from changes in water film thickness and the probability of ice formation. Variations in water film thickness are further adjusted to account for pore structure distribution during freezing and thawing, leading to a new model for predicting UWC. Our results demonstrate that supercooling is influenced by the initial contact angle, which is controlled by pore size, particularly for pores smaller than 2 μm. Differences in water film thickness between freezing and thawing generate UWC hysteresis, which is governed by changes in the contact angle. The accuracy of the model is validated against experimental data, effectively describing SFCC and STCC under varying freeze–thaw cycles. Additionally, the UWC model is applied to simulate unidirectional soil freezing, clearly illustrating the effect of supercooling on soil frost heave.</p>