The soil freezing characteristic curve (SFCC) is a mathematical description on the relationship between the unfrozen water content and the freezing temperature in frozen soil. It is an important basis for the research of hydrothermal migration

frost heaving and constitutive relations of frozen soil. The empirical expressions of SFCC are widely used in the literature

while the investigation on its physical understanding is relatively rare. By considering the adsorption and capillary interaction between the soil particles and the ice interface

a new SFCC model for saturated frozen soil is proposed from the pore scale based on the equilibrium pressure of ice water phase transition composed of adsorptive and capillary pressure. The calculation results of the model show that the unfrozen water content will decrease continuously at the same temperature

and the SFCC will become steeper as the particle radius is gradually increased. The SFCC of monodisperse silica microspheres was tested based on a nuclear magnetic resonance equipment. The experimental results were compared with the mathematical model

which showed that the proposed SFCC model is in good agreement with the experimental results. The new SFCC model manifests a clear physical meaning and provides a theoretical basis to reveal the mechanism of frozen soil.