This study conducts a theoretical analysis of unfrozen water content in unsaturated soil below freezing temperatures and proposes a simple prediction method with its mathematical model. By considering the chemical and mechanical equilibrium of gas–liquid phases in unsaturated soil pores, along with the soil-water characteristic curve, the soil pore volume distribution density function is derived using the parameters from the Van Genuchten soil-water characteristic curve model. The results indicate that water freezing is closely related to the soil pore size distribution. The relationship between the maximum pore size filled with liquid water at initial effective saturation and initial crystal pore size at a specific freezing temperature is discussed to analyze ice-water phase transition characteristics in soil pores and critical effective saturation. Freezing occurs only if initial effective saturation surpasses critical crystallization saturation. A formula for predicting unfrozen water saturation in unsaturated soil below freezing temperatures is provided and validated against existing experimental data.