In order to improve the ability of a reservoir to resist flood, increase the time of flood dispatching and warning, and ensure the safe operation of reservoir, a new method called reservoir upper limit initial water level analysis based on torrential rain classification is proposed. Firstly, the total amount range of rainstorm of certain duration is analyzed, and the possible rainstorm is classified according to rainfall. Then the spatial and temporal distribution of rainstorm at different levels is simulated. The hydrological model is used to simulate the flood process and the flood control calculation is carried out according to the original scheduling method. And then the safe-operating ideal storm volume is determined. On this basis, the extreme heavy rain exceeding the safe-operating ideal storm volume is further studied and the corresponding upper limit initial water level is calculated. It can provide data reference for reservoir pre discharge operation and increase flood control warning time, so as to ensure that the highest water level in flood routing does not exceeds the allowable upper limit water level of the reservoir. The method is applied to the Shiyazi reservoir. The safe-operating ideal storm volume of the reservoir under the original dispatching method is analyzed and calculated. The safe-operating upper limit water level corresponding to the heavy rainfall exceeding the rainstorm amount is analyzed, so as to provide data support for flood control of Shiyazi reservoir. It is found that the flood control operation of some small and medium-sized reservoirs has obvious characteristics, the capacity of resisting extreme flood is limited according to the original operation mode and the new method can effectively improve the ability of the reservoir to resist the catastrophic flood.