For seepage in unsaturated soil, there are both air flow and water flow, which can be called the water-air two-phase flow. In order to simulate the water-air two-phase flow in soil when there is groundwater, a numerical model of water-air two-phase flow in saturated-unsaturated soil is established in this paper. By the model, the air-flow and water-flow in unsaturated soil are both considered in seepage calculation. And the mass transfer between air-phase and water-phase, change of phase states are considered in calculation. Capillary pressure is the most important factor for the water-air two-phase flow in unsaturated soil, and the calculation method of capillary pressure is also given in the paper. At last examples are given to verify the correctness of the numerical model and the calculation method.