The computation of infiltration rate is a key step for estimating the infiltration excess runoff in dry and semi-dry regions. The infiltration equation used in Hebei model is made of two parts, the one is the infiltration rate governed by tension and the other is that governed by gravity (the constant infiltration capacity). Because of the repetition of rainfall rate used to compute the two infiltration rates, the total infiltration rate is always greater than it should be and as a consequence, the runoff amount and the peak discharge of the hydrograph simulated by the hydrological model is decreased. A modified infiltration equation is presented, in which the rainfall rate used to compute the infiltration rate governed by tension is the difference between the total rainfall rate and the constant infiltration capacity. Computation for the modified infiltration equation and the original?one under conditions for rainfall rates ranging from 2 mm/h to 10 mm/h and initial water content ranging from 0 mm to 140 mm are carried out, and the results from the two infiltration equation are compared. It can be seen from the results of the Dage watershed by the modified infiltration equation that the runoff amount and the peak discharge of the hydrograph simulated are increased 14.14% and 10.08% respectively.