The invention discloses a double-mode flash flood warning control system and a method, which belongs to the technical field of disaster warning. The invention especially relates to a method of building a flash flood warning control system based on critical rainfall and soil moisture saturation, and particularly relates to a flash flood disaster comprehensive research and judgment and warning control system coupling critical rainfall and soil moisture saturation. Workstations are built in flash flood-prone areas or geologically fragile areas, wherein there are no flash flood hazards near the workstations. A soil moisture measuring instrument, a data transmitter, an automatic rainfall recorder, an automatic weather station, a GPS base station and other facilities are arranged in each monitoring area. The system couples critical rainfall and soil moisture saturation. The possibility and magnitude of flash flood are determined according to critical rainfall and soil moisture saturation respectively. A single standard is changed to double guarantee, and the sensitivity and accuracy of monitoring are improved. The system and the method can be widely applied to hilly areas, and realize more accurate, timely and efficient monitoring and warning on flash flood disaster.