A physics-based infiltration capacity distribution curve, which was calculated using the soil and rainfall data, was introduced into the Green-Ampt rainfall-runoff model to improve its performance. The improved Green-Ampt model was compared with the traditional Green-Ampt model and the Xin’anjiang model in two semi-arid basins in China and US, and then, the performance and applicability of the improved model in semi-arid regions were evaluated. The results show that in two semi-arid basins, the improved Green-Ampt model performs better than the traditional Green-Ampt model and the Xin’anjiang model. The added physics-based infiltration capacity distribution curve effectively reflects the changing of infiltration capacity with time and rainfall for the whole basin, and the results of infiltration and runoff generation calculated by the improved infiltration curve are more accurate. The flood in semi-arid region is mainly caused by the short-term and heavy rainfall, and the runoff is dominated by the infiltrated-excess surface runoff. Rainfall observation accuracy and rainfall intensity have great influence on the simulation accuracy of hydrological models. The improved Green-Ampt rainfall-runoff model can play an important role for the flood forecasts in semi-arid regions.