To reasonably activate the semi-restoration polder (SRP) and detention and retention area (DRA) and effectively reduce the water level of Poyang Lake, a two-dimensional hydrodynamic model of Poyang Lake and the downstream sections of its five tributaries was established using MIKE21. The extreme flood event was simulated by amplifying the 2020 Poyang Lake flood using the same-frequency method. Nine combination schemes were formulated by setting different activation water levels for SRP and DRA, and the flood diversion processes were simulated. The optimal scheme and its associated economic losses were further analyzed. The results indicate that the optimal flood diversion effect is achieved when the SRPs are activated at water levels of 21.50 m (for protection areas under 10 000 mu) and 22.30 m (for protection areas above 10 000 mu) and the DRAs are activated at water level of 22.50 m. Under this scheme, the flood diversion volumes for the SRPs and DRAs are 2.617 billion m³ and 2.569 billion m³, respectively, with maximum 24-hour water level reductions of 0.37 m and 0.25 m, respectively. After 48 hours of activation, the direct agricultural economic loss for the SRPs is 209 million yuan, while the loss for DRAs is 417 million yuan, of which the loss for the Kangshan DRA is 154 million yuan.