Abstract: The flash flood warning system in small basin is the most important non-structural measure for disaster prevention. Flash flood disasters occur rapidly, and it is hard to forecast rainfall and water level. To reduce the disaster damages, an urgent task is to develop an accurate flood forecasting system for small basins. This paper introduces a rainfall warning indexes analysis method based on survey results of flash flood disasters. The flow is calculated by water level, and a rainfall warning indicator is introduced by flow. The aerial precipitation in small basin is calculated using Thiessen Polygon method. With the aerial precipitation and the rainfall warning indexes, the threshold l rainfall index of flash flood early warning system is created accurately.