The soil-water characteristic curve (SWCC) plays an important role in the prediction of soil performance. To predict the water film thickness on the surface of grains, a chemical model is proposed by considering the effects of van der Waals forces and electric double-layer forces. Then a liquid bridge model is proposed to predict the relationship between water content and matric suction of two contacted spherical grains. By studying the microstructure and incorporating the liquid bridge model into the representative volume element of soils, the SWCC of soils is derived and the volume of water films corresponds to the residual water content. The contact angle hysteresis is considered to capture the SWCC hysteresis, and the concept of equivalent grain radius is proposed to consider the effect of grain-size distribution. By comparing test data and model predictions, it is found that the proposed model captures the SWCC of soils well. Model predictions also reveal a new swelling and shrinkage mechanisms of expansive soils. At a low water content, the suction is huge for fine grains, and this causes the shrinkage of expansive soils. When the water content approaches to be saturated, the matric suction is negative and it forces the separation of contacted grains. This contributes to the swelling of expansive soils. It also reveals that the change of SWCC after wetting-drying cycles arises from the microstructure change of soils. At a given water content, the matric suction decreases with the increase of porosity.