Abstract: Mountainous flash flood disaster prevention and control area in China accounts for 40% of the whole land area. Flash flood prevention is an urgent need for disaster prevention and reduction in mountainous areas and economic and social development, and it is an important guarantee for preventing people from returning to poverty due to disasters and consolidating China's achievements in overall poverty alleviation. Risk assessment and early warning is an efficient tool to prevent life and economic loss by flash flood disaster. Current framework of flash flood risk assessment and early warning system mainly adopts runoff production estimation based on hydrological methods of the relationship between rainfall, discharge and water level. However, it should be noticed that flash flood produces not only runoff but also huge sediment, which together determine the inundated area, scope, early warning indicators and warning time. It may highly underestimate the flash flood risk if not considering sediment influence, resulting in missing and misreporting of disaster warning and reducing the effectiveness and reliability of flash flood disaster prevention. Focusing on sediment influence on flash flood risk, key factors such as sediment concentration and sediment yield way is identified, and the influence law of sediment on flood and sediment disaster risk was analyzed. Combined with the existing watershed sediment yield model and monitoring technology, sediment risk was introduced into runoff risk. Different early warning indexes and thresholds for runoff and sediment risk are selected to achieve rapid, accurate and comprehensive assessment and early warning of flash flood with the influence of sediment impacts which can provide theoretical and technical support for flash flood disaster prevention in China.