Rainfall infiltration is a process of water flow driving air flow through unsaturated soil

which is a difficult to exact simulate numerically in geotechnical engineering and the influence of pore air is ignored. In this paper

a water-air two-phase flow model is proposed based on the water-air mass balance equations

Darcy’s law and multiphase flow theory. The integral finite difference method and Newton-Raphson iterative method are used to solve the water-air two-phase model. By switching primary variables

a set of phase state (single phase and two-phase) are taken into account. The refined treatments of every boundary condition are described in detail. The water-air two-phase flow model is verified by simulating an experiment of water through an earth column. The process of rainfall infiltration is simulated numerically. The variation of pore water pressures

pore air pressure

capillary pressure and saturation of water and air phase in different depth of the soil are described; and it is verified that the water-air two-phase flow model is a better model to perform transient seepage problem; and airflow will obstruct water flow from infiltrating into the soil.