There are numerous cascade reservoirs in China’s small and medium-sized river basins. Once the cascade reservoirs break, the disastrous consequences will result in extreme economic, social, and ecological losses. According to the hidden danger investigation results of cascade reservoirs, a risk assessment method and standard for cascade reservoirs were proposed and established based on the risk matrix method, which provides a foundation for the risk assessment of cascade reservoirs. Taking the D, C, and H cascade reservoirs as the case study, according to the hidden danger investigation results and dam break consequences of the reservoirs, it was concluded that the risk levels of the three reservoirs were in order of extremely high risk, high risk, and high risk. Therefore, it is suggested that engineering and non-engineering measures should be taken to reduce the cascade reservoir’s risk to ensure the high-quality and safe operation of the cascade reservoir group in China.