The modified Green-Ampt infiltration equation which is suitable to describe infiltration during an unsteady rain is widely used to simulate infiltration and runoff on the slope land.The average potential across the wetting front S_f is one of the most important parameters in it, whose validity is dependent on S f .The theoretical relationships among the parameters of the Philip equation and Green-Ampt equation are analyzed,and parameter S f in Green-Ampt equation can be calculated by the parameters values of the Philip equation. The infiltration experiments with surface ponding and unsteady rain were carried at the Changwu A gri-ecological Station in 1996, and the different values of S_fcalculated by Green-Ampt equation and parameters of the Philip equation were obtained based on the infiltration data w ith surface ponding. The validity of different S_f'_s values was compared by the runoff data of unsteady rain on a plane via the modified Green-Ampt equation by Chu (1978).The results showed that S f calculated by the parameters of Philip equation was valid for simulating the infiltration and runoff on a plane on the Heilu soil of theLoess Plateau.