The mountain flood disaster is one of the major natural disasters in the world. The global economic loss caused by flash floods in the 21st century has reached more than 46 billion US dollars per year. The area of mountain flood disaster prevention and control accounts for about 40% of the land in China, whereas the fatality caused by mountain flash floods account for approximately 70% of the death toll from flood disasters. In recent years, the construction of mountain torrent disaster prevention and control projects have been carried out in an all-round way in China. A mountain torrent disaster prevention and control system combining specialists and groups has been basically established, and the technical level of mountain torrent disaster monitoring and early warning has been significantly improved. Examples of torrential rain and flash flood disasters show that the flash flood disasters with heavy casualties and property losses often result from the combined effects of flood and sediment. In the past, scientists mostly paid attention to the flood's role while ignoring that the combined effect of flood and sediment would significantly increase the risk of mountain torrent disasters. To further improve the ability to prevent and control mountain flood disasters, it is urgent to study the critical technologies of flood and sediment disaster forecasting and early warning. The project “Research and Demonstration of Key Technologies for Forecasting and Early Warning of Flash Flood and Sediment Disasters in Mountainous Rainstorms” focuses on the joint action of flood and sediment and condenses four critical scientific and technological issues: 1) mutation mechanism of runoff and sediment yield and disaster-causing coupling mechanism of the flood-sediment process and gully-bed drastic change in mountainous area under heavy rain; 2) early identification of flood and sediment disasters and integrated intelligent monitoring technology for disaster-causing elements in mountainous rainstorms; 3) simulation and rapid prediction technology of flash flood and sediment movement process in a mountainous rainstorm; and 4) disaster risk dynamic assessment and early warning and prevention technology based on the dynamic process of mountain flood and sediment disaster. Focusing on the connotation of critical scientific and technological issues, we proposed five key research contents including 1) study on the process of runoff and sediment production in mountainous areas and the disaster mechanism of flood and sediment coupling; 2) early identification and intelligent monitoring technology for flood and sediment disasters in mountainous areas; 3) simulation and rapid prediction technology of flood and sediment movement process in a mountainous rainstorm; 4) dynamic assessment and early warning technology of flood and sediment disaster risk in mountainous areas; 5) construction and demonstration of a platform for forecasting, early warning, and prevention of flood and sediment disasters in mountainous areas. The research results will improve the real-time accuracy and intellectual level of monitoring, early warning, prevention and control of heavy rain and flash flood disasters in China.