Sichuan Province in China is characterized by complex terrain and abundant water resources, with a spatiotemporal distribution of precipitation that is notably non-uniform. Short-term heavy rainfall during the flood season often leads to flash flood disasters, posing significant threats to both lives and property. It's always been difficult to calculate the critical rainfall of mountain flood in mountainous small watershed where water level and discharge data are lacking. In this study, Luba River, the upper reaches of Qujiang River Basin in northeast Sichuan Province in China, is selected as a pilot research area to simulate the flooding of a precipitation process that has caused the most obvious flooding disaster in recent years. Hourly precipitation data in September 2014 were used in this study. The Floodarea Hydrologic model, a two-dimensional unsteady hydrodynamic model developed by German Geomer, was adopted for simulation. In this paper, combined with the information of field investigation, we simulate and reproduce the inundation caused by precipitation in this area. The simulation results achieve a good consistency between the simulated inundation depth of the early warning point and the measured depth of the field survey. The results show that FloodArea model can be applied to the analysis of rainstorm and flood disaster and the study of torrential flood risk rainfall in small watersheds with no hydrological data represented by the Luba River Basin. In addition, we also calculated the critical rainfall threshold of different risk levels of warning points, which can provide some reference for the disaster prevention and mitigation of flash floods in this region.