Abstract: To realize the flood risk warning of small reservoirs in ungauged regions, a rainfall retaining capacity calculation model considering different operation modes of small reservoirs and the model parameter schemes have been developed in this study.An iterative trial solution method considering potential rainfall for the constructed model has been proposed for efficient computation.The rainfall retaining capacity of32pilot small reservoirs located in the ungauged region of Hu′nan Province was calculated starting from August21,2023.In addition, a continuous calculation was carried out from August1to September30,2023,using the proposed method for Heping Reservoir.The results show that the flood risk of the pilot small reservoirs calculated is reasonably related to the rainfall distribution, supporting flood risk visualization for small reservoirs within the rain-affected region.Furthermore, the impact of reservoir water level and antecedent precipitation on rainfall retaining capacity can also be dynamically quantified in real-time, which provides a reference for the continuous monitoring of flood risks of small reservoirs.