A theoretical model for predicting the soil-water characteristic curves of simple granular soils during different wetting and drying processes is proposed. In this model, the irregular pores in a simple granular soil are assumed to be regular symmetrical cone-shaped capillary pores. The wetting and drying processes of the soil are considered to be the filling and draining processes of the symmetrical cone-shaped capillary pores. The degree of saturation of the soil is determined by estimating the amount of water in the cone-shaped capillary pores. The suction in the soil is estimated by applying the so-called capillary law to the cone-shaped capillary pores. The influence of the closed air bubbles in pore water on the soil-water characteristic curves is also simulated in the theoretical model. All model parameters in the model have definite physical base. The model is verified using experimental data.