Unfrozen water affects the thermal-hydro-mechanical characteristics, microbial activity and freeze-thaw processes in frozen soils. This study found that part of the unfrozen water is formed by the coupling of adsorption and capillary action in soils with mid to low clay content, which is called bound-capillary water and located in nanopores between clay and sand (silt) particles. The nature of the bound-capillary water affects soil freezing characteristics, which varies with the initial water content. The influence of coupling effects arises from the adsorption effects on the clay surface and the capillary action between the clay-sand particles. The adsorption effects (or surface effects) establish an electrical double-layer structure for bound-capillary water. The capillary action (surface tension) forms bound-capillary water in a shape of meniscuses, which significantly increases its content. Here, we established four theoretical models and a parametric model of unfrozen water based on the coupling effects. The basic mathematical expressions of four theoretical models are almost identical to those of 10 existing unfrozen water semi-empirical models, demonstrating that the semi-empirical models are representative of empirical formulas describing the freezing characteristics of the bound-capillary water. A comparison of model results with the measured unfrozen water content of 9 soils verifies that the parametric model is suitable for soils with low to medium clay content.