Abstract: The flash flood caused by extreme rainfall has become one of significant obstacles that highly affected the social economic development.In order to scientifically understanding the mechanism of flash flood disaster and to produce technological support to the work of local flood prevention and control,the FFMS distributed hydrological model has been applied to assess the disaster process.Taking the “8·12” flash flood disaster at Liulin County of Hubei Province as an example,the analysis was made based on the numerical simulation of flood sources and runoff component.The results show that the spatiotemporal variable source mixed runoff modelling approach is able to accurately represent the flood process and the variation of runoff component during the flood duration,and to clearly reveal the disaster mechanism.The main cause of this disaster is due to the flow peak superposition which consisted by the three flood peak came from different tributaries met at same time at the Liulin County.Moreover,the peak flow of the Lianhua river at the downstream also arrived at same time which strongly affect flow progress and enhance the flood damages.