Abstract: Small and medium-sized reservoirs in China have the characteristics of large number and wide distribution, fast confluence speed, small flood storage capacity and short emergency response time, and it is of great significance to ensure its safety in flood season. The concept and calculation method of small and medium- sized reservoir's rainwater retaining capacity are introduced, and the runoff coefficient method and hydrologic model method are used to calculate the rainwater retaining capacity of Fanbuqiao Reservoir, the calculation results of runoff coefficient method are compared with the calculation results of hydrologic model method and actual situation. The results of real application show that: (1) Calculating reservoir's rainwater retaining capacity before rainfall process can effectively prolong the flood forecast period; (2) The runoff coefficient method is simple and convenient, and the results have a certain accuracy. However, this method is empirical and does not consider the influence of soil moisture content, reservoir discharge and other factors, the results change greatly, so it is not suitable for reservoir early warning service; (3) The hydrological model method fully considers the influence of the underlying surface of the basin, rainfall process, current water level of reservoir, current discharge flow and other factors, so the calculation results are more reasonable.