[Objective] In this paper, we aimed to determine the critical rainfall of flash flood in Tibet, in order to provide the basic data for hydrological and meteorological warning in Tibet and serve the flash flood prevention. [Methods] We simulated the process of a flood in Southeastern Tibet by hydrodynamic model Floodarea, and calculated the submergence depth by using meteorological data of hourly precipitation, and we then obtained the critical rainfall of flash flood. To test the simulation result, we conducted a field survey to obtain the basic parameter of a flash flood disaster in August 2015 in Bayi Nyingchi region. [Results] The Floodarea model could be well used in the simulation of rainfall induced mountain flood process. The critical rainfall of flood submerged in different water depth could be calculated by adjusting the rainfall scenarios. According to the relationships between rainfall amount and submergence depth, the critical 6 hours accumulated rainfall amounts in four mountain flood ditches with different submergence depths(0.1, 0.5, 1.2, 1.8 m) were 33, 55.7, 75.4 and 91.9 mm, respectively. [Conclusion] Floodarea model software can be applied in the complex terrain mountainous areas that lack of hydrological observation data, in Tibetan Plateau. It can accurately simulate the flash flood disaster process and determine the critical rainfall. This study can provide reference for flood prevention and early warning in complex terrain mountainous areas.