Abstract: Taking county as the base-unit, a flash flood disaster risk assessment method based on the basic data of previous investigation and evaluation is put forward. This paper analyzes the disaster-causing factors and disasterbearing factors of flash flood disaster extracted and processed by GIS tools. The weight of each factor is calculated by AHP(Analytic Hierarchy Process). Finally, the risk maps of flash flood disaster are developed by overlapping the layers based on the weighted synthesis method, which is verified by the historical data. Taking Sanya City as an example, the results show that the risk level of flash flood in the basins of Tengqiao River, Sanya River and Ningyuan River is higher, and there are the high incidence and vulnerable areas of flash flood.