The variation of unfrozen water content with temperature has a significant effect on the coupled heat-water transport in freezing soil, which can cause the frost heave and thaw settlement, and thus influence the stability of infrastructures. The premelting theory for water-ice in soils is developed to study the unfrozen water variation in freezing soil. The developed theory integrates the interfacial premelting of contact ice, the soil particles, the melting of the ice surface, and the premelting induced by impurity and curvature. A model to predict the unfrozen water content is then established by considering the change of unfrozen water film. The equivalent grain size is introduced to improve the solving efficiency in the calculation. Finally, the proposed analytical model is verified by the test data. The results indicate that the thickness of water film in soils increases when the surface charge density and the impurity concentration increase under the same supercooling degree. The surface melting on the interfaces of soil particle and the pore ice has key influence on the variation of the liquid water content. Meanwhile, the unfrozen water content increases with the increasing impurity concentration and surface charge on soil particles. Besides, the unfrozen water content also increases with the decreasing radius of soil particles.