The soil-water characteristic curve (SWCC) of a low-plasticity clay over a wide range of suction was determined based on the pressure plate and vapor equilibrium techniques. The soil samples were prepared by saturating the low-plasticity clay with NaCl solutions with different concentrations. The effect of pore solution concentration on the SWCC was analyzed. The experimental results show that the effect of salt content has little influence on the matric suction, and has great influence on the total suction, the reason for this is that the saline solution induces the osmosis suction. In the vapor equilibrium experiments, it is found that the solution concentration increases with the decrease of water content, and correspondingly the osmosis suction increases. With the decrease of water content, however, the matric suction increases rapidly, while the contribution of osmosis suction decreases gradually. In unsaturated soils, the total suction (the negative value of pore water potential) includes the matric suction and the osmosis suction, where the matric suction has two contributions, i.e. capillary and adsorption, and the osmosis suction depends on the concentrations of pore solution. When the water content of the sample is low, the adsorption effect is dominant. Based on the experimental results, the interaction between adsorption water film and the soil particles is analyzed, and the influence of the solute on the intermolecular forces is also investigated. According to the surface chemistry theory, a relationship between intermolecular forces and adsorbed water film is proposed to describe the soil-water characteristics at low water potential.