A simulation study of the infiltration rate was conducted for four soils with different rain patterns. Computational results showed that the rain water was infiltrated into soil when the infiltration rarer was greater than the rain rate at the earlier stage of the rain, and then the infiltration flow was calculated by analogy with saturated piston-flow, the infiltration rate was in direct proportion to the suction head at the wetting front, a front of demarcation between saturated and unsaturated zones, and in inverse proportion to the saturated zone length. With the,definition of nondimensional suction head equal to the suction head divided by its maximum one, the nondimensional heads calculated from the theory of Mein and Laison and predicred by the sitnelation study were in goad agreement.By using the piston-flow concept,the method to determine the ponding time and wetting front depth has been derived in this study. Also, the analytical solution for infiltration rate and wetting front location below the soil surface, during the period when rain intensity is alwayes greater than infiltration, has been obtained.