The hydro-mechanical coupling is the basic characteristics to the unsaturated soil, especially to the one which consists of clay particles. Pore structure of clay is sensitive to the water content; and the pore size distribution (PSD) changes at every moment during drying process. The evolution laws of microstructure under the hydraulic path are first summarized based on experimental results. Subsequently, a novel model is developed by relating the pore-size distribution to the void ratio. According to the model, the PSD can be transformed from the initial PSD through three steps: translation, scaling and dispersion. The quantity of displacement and scaling are linear with void ratio; and the dispersion is exponential attenuation with void ratio. Finally, the model predictions are compared with experimental measurements in the literature, showing that the new model is capable of describing the microstructure evolution of unsaturated soil during drying process.