•An unfrozen water content model is developed based on the pore size distribution.•The content of micro-pore has a significant effect on the three parameters.•The model could simulate the change of unfrozen water content in freezing soils.•An expression is developed for calculating the thickness of unfrozen water films. A clear fundamental understanding of the soil freezing characteristic curve is crucial for studying soil freezing behavior. In this paper, based on the assumption that the shape of the soil freezing characteristic curve is mainly dependent upon the pore-size distribution of the soil, a mathematic model for estimating the soil freezing characteristic curve is proposed. The formula has the form of an integrated frequency distribution curve, which is verified by previous researches (seven representative soil samples and six representative mineral compositions, a number of special mineral particles and soils, and unsaturated soils). By nonlinear curve fitting, the correlation coefficients are generally larger than 0.95. The proposed model is more convenient than the original empirical formulas in numerical modeling, and it can overcome the shortcoming that the original empirical formulas are not derivative at the temperatures near the freezing point. In addition, the proposed model directly expresses the relationship between residual unfrozen water content and temperature under extremely low temperature conditions. Of course, the new model and results in this study may provide a reference for the research on basic physical properties of freezing soils, and the related numerical modeling in cold regions engineering.