Abstract: In order to improve the accuracy of mountain torrent simulation and forecast in Zijingguan Basin, the critical rainfall in Zijingguan Basin is inferred, so as to provide reference for mountain torrent forecast and early warning. HEC-HMS hydrological model of Zijingguan Watershed was constructed based on the measured rainstorm flood data and underlying surface data of Zijingguan Basin. With typical rainfall processes of different scaling ratios as input,the critical rainfall values under different soil moisture conditions were calculated by running a large number of models. The results show that the relative errors of simulated flood peak and flood volume in the periodic and verification periods of the HEC-HMS model are all less than 20%, which c meet the accuracy requirements. the HEC-HMS model can be used for flood simulation and flood warning in Zijingguan Basin. Soil moisture condition in the early stage of the basin is an important factor affecting the critical rainfall, and the critical rainfall is also different under different soil moisture conditions in the early stage of the basin. The research results have important guiding significance for improving the calculation method of mountain flood disaster early warning and prediction.