Abstract: Reservoir operation based on forecasting is one of the important ways of flood operation of reservoir and can improve the flood prevention ability of reservoir basin. Miyun Reservoir, which has set a new water storage record, will operate at a medium and high water level for a long time and the flood discharge capacity of the downstream river of the reservoir is limited, which increases the flood defense pressure of the reservoir. In order to effectively solve the dilemma of "storage and discharge" in the upstream and downstream of the reservoir, this paper analyzes the operation of the Miyun Reservoir, the development of flood forecasting, the impact range of flooding in the upper and lower reaches of the reservoir, and the example of operation based on forecasting. It is concluded that the flood forecast system of Miyun Reservoir based on the distributed hydrological model EasyDHM can well adapt to the flood simulation in the basin of Miyun Reservoir. The flood forecast results can support the predischarge operation of reservoir well.