In this study, numerical simulations were carried out to analyze the effects on flood diversion by the Garden Lake in the case of a flood with ahundred-year return period. The results show that when the Garden Lake is in operation, that is to say, at the time that the flood gates are kept open, the Garden Lake flooding area plays a role of cutting the peak of discharge in the upstream of Huaihe River and shifting the peak of water level in the downstream of Huaihe River. In addition, the extreme values of flow velocity in the Garden Lake flooding area appear near the downstream sluice gate. Especially at the beginning of the inversed discharging flood, the flow velocity at the downstream sluice gate can reach up to 0. 7 m/s. Last but not the least, the results reveal that properly delaying the operation of Gargen Lake flood area, despite of increasing the maximum water level of mainstream of Huaihe River, can effectively slow down the inundating process of the flooding area of the Garden Lake, which is conducive to improving the ability of the Huaihe River to regulate and store floods.