Abstract: To simulate the fluids(water and air) flowing through unsaturated soils,a numerical model for water-air two-phase flow through unsaturated soil was developed.Water and air phases were assumed to occupy the entire pores of porous media simultaneously.Flow of water and air through unsaturated soils could be explained by balance equations of mass and extended Darcy’s law in unsaturated soils.An integral finite difference algorithm was presented for numerically solving the governing equations of water-air two-phase flow in saturated-unsaturated porous media.Nonlinearities were handled by the Newton-Raphson iteration.A computer code named SEEP2P was developed by the authors to perform fluids flow simulation in saturated-unsaturated soils.Examples were presented to demonstrate the capabilities of the model.It was indicated that the single-phase approach to unsaturated flow was accurate enough for steady state seepage,but the multiphase flow of air and water must be considered in unsteady state seepage like rainfall infiltration problem.And flow of water and air simulated by numerical model of water-air two-phase flow could be used to quantitatively analyze the influence of air pressure on infiltration of water into soil.