China has a lot of small- and medium-sized reservoirs located in ungauged hilly areas, most of which has small catchment area and short response time in face of flash floods. Early warning of the retaining capacity is one of the important nonstructural measures to improve the flood control capacity of small- and medium-sized reservoirs during flood season. The concept of small- and medium-sized reservoir’s retaining capacity was proposed and explained systematically for the first time in this paper, and the calculation method was described step by step in one studied reservoir. This study shows that the concept of retaining ca - pacity can be used as an early warning method to extend the leading time of flood forecast and so to improve the small- and medium-sized reservoir’s flood control capacity.