Abstract: The Haihe River Basin experienced an extreme rainfall event in late July 2025 which caused the largest recorded flood in the Miyun Reservoir since its impoundment in 1959. A digital watershed model of the Miyun Reservoir Basin was constructed to simulate the "25·7" flood process and analyze the runoff yield and confluence characteristics. The simulation results showed that this flood exhibited a distinct "double-peak" hydrograph. The first flood peak occurred on July 27th, with a simulated peak inflow of 6 323 m³/s (observed 6 550 m³/s) on baseflow of prior rain-runoff release. The second peak occurred on July 28th, with a simulated flow of 4 823 m³/s (observed 4 550 m³/s). The cumulative inflow volume over seven days reached 0.974 billion m³ (observed 0.91 billion m³), corresponding to a recurrence period of exceeding 100 years. The simulation results indicate that antecedent precipitation and soil moisture retention significantly enhanced the runoff coefficient during successive storms. Therefore, when applying hydrological analysis methods to calculate runoff yield, it is crucial to account for antecedent rainfall conditions and make necessary adjustment of runoff coefficient to avoid underestimating peak discharges.