The early warning model for flash flood was based on the hydrological and geomorphological concept to the river basin, with the principle that flash flood will only occur where there is a high potential risk and when heavy rainfall exceeds a threshold. In the model to build flash floods map risk, the parameters of the basin were analysed and evaluated, and the weight was determined by the method of the hierarchical analysis (Thomas Saaty-Analytics Hierarchy Process - AHP). The flash flood early warning software was built on the basis of open source programming tools. With the spatial module and online processing in the internet, early predicted precipitation of 1-6 days for the IMETOS automatic meteorological stations would be interpolated, then processed with the potential risk maps. The processed results would determine the locations of flash flood at the several risk levels corresponding to the predicted rainfall values at the meteorological stations. The system was constructed and applied to flash floods disaster early warning for Thuan Chau district - Son La province when the rainfall exceeded the 150 mm/day threshold.The system has initially supported the positive decision making to prevent and minimize damages caused by flash floods.