Early warning of the flash flood risk in mountainous areas is an important requirement today for developing countries like Vietnam. Most of the analytical methods require large amounts of quality data and complex analytical models, making it difficult for users. Therefore, it is necessary to have a flash flood risk analysis tool to give an early warning alarm simply, so that people living in high-risk stream basins such as Nghia Do, Bao Yen district, Lao Cai province can be more proactive in their actions. This paper aims to use simple statistical methods to assess the flash flood risk in Nghia Do stream. The research team used statistical methods to explain both spatial and temporal laws of flash flood formation. Research results show that the development of flash floods on Nghia Do stream has a fast and sudden speed, mainly depending on the rainfall intensity in 24 hours and just 24-hour rainfall intensity higher than 29mm is enough to form flash floods. This study establishes a 2-dimensional matrix table combining 5 potential levels of flash flood formation with 5 levels of rainfall intensity in 24 hours to determine the risk of flash flood formation in the area. People here can easily use this matrix to warn themselves in time of the risk of flash floods occurring in the Nghia Do area.