The Green-Ampt model is a simple method for infiltration analysis. Due to its ease to use, the Green-Ampt model is potentially very useful for the prediction of slope failure under rainfall infiltration. A key parameter in this model is the permeability coefficient, which is different from the saturated permeability of the soil. This study investigates the relationship between the permeability coefficient in the Green-Ampt model (kg) and the saturated permeability coefficient of the soil (ks) through comparison with results from the more accurate Richards’ equation. The ratio of kg to ks is called the permeability coefficient adjusting factor. It is found that the adjusting factor depends on the factor of the duration of the rainfall as well as the soil type. For the soil studied in this paper, an adjusting factor of 0.7 seems to be optimal in reproducing the pore pressure distribution calculated based on Richards’ equation.