The presence of unfrozen water within frozen soil is the primary factor responsible for the phenomenon of segregation frost heaving. Variations in its quantity directly affect the soil's pore distribution. In this study, saturated silty clay was chosen as the subject of investigation. A nuclear magnetic resonance (NMR) testing apparatus was utilized to measure the unfrozen water content in the soil at various temperatures and dry densities under freeze-thaw cycle. By analyzing the transverse relaxation time T2 distribution curve, a qualitative and quantitative assessment of the soil's pore distribution throughout the freeze-thaw process was conducted. Considering that the most established models were empirical, a characteristic curve was developed to illustrate the changes in unfrozen water content during freeze-thaw cycle, based on capillary theory and the Gibbs-Thomson equation. The least square method and experimental results were used to ascertain the model parameters and to verify the efficacy of the freeze-thaw characteristic curve. The analysis revealed a pronounced hysteresis effect in the unfrozen water content curve during freeze-thaw cycle. At the same sub-zero temperature, as dry density increases, the unfrozen water content in frozen soil tends to decrease, and the pore distribution undergoes significant alterations with temperature fluctuations. The proposed freeze-thaw characteristic curve can accurately predict the changes in unfrozen water content with temperature under various testing conditions, which is crucial for forecasting freeze-thaw deformation in engineering projects located in cold regions.