[Objective] More than 700 floodplains and polders are distributed along the mainstream of the middle and lower reaches of the Yangtze River, with a total flood storage capacity of about 16.41 billion m³. These areas serve as important spaces for flood discharge and storage of the Yangtze River and are also home to millions of people. Difficulties in operation during major floods, insufficient safety guarantees during ordinary floods, and a lack of management policies have become the most notable weak links in the Yangtze River flood control system. Existing studies lack in-depth investigation into the operation sequence and activation timing of floodplains and polders. They also do not thoroughly examine the differences in flood discharge and storage effects arising from various combinations of different types of floodplains (mid-channel bars and outer floodplains). This study constructs a two-dimensional unsteady flow mathematical model to quantitatively analyze the flood diversion effects under different activation water levels and operation modes, aiming to provide a scientific basis for the hierarchical optimal operation and flood control management of floodplains and polders. [Methods] The lower Jingjiang reach and the Hukou-Datong reach were selected as typical areas. A two-dimensional unsteady flow mathematical model was established based on MIKE 21, and the flood evolution processes of floodplains and polders under different operation modes were simulated under the 1954 flood condition regulated by the Three Gorges Reservoir and the upstream reservoir group. [Results] The operation of floodplains and polders effectively reduced short-term flood water levels. In the lower Jingjiang reach, the maximum water level reduction at Shishou and Diaoguan stations reached 0.44 m and 0.36 m, respectively, and the duration with reductions exceeding 5 cm lasted about 5.7 days. In the Hukou-Datong reach, the maximum water level reduction at Balijiang, Anqing, and other stations ranged from 0.07 to 0.16 m, and the duration with reductions exceeding 5 cm lasted 1.4 to 3.2 days. After the floodplains were fully filled, the peak-reduction effect was significantly weakened. The backflow of stored water during the flood recession period caused a slight rise in water level (about 1-4 cm). Comparison of different activation water levels showed that when the activation water level in the Hukou reach was increased from 20.5 m to 22.0 m, the maximum water level reduction along the reach increased by 0.07-0.13 m, but the duration with reductions exceeding 5 cm was shortened by 8-24 hours, indicating that activation at a higher water level could cope with more severe floods but resulted in a shorter duration of water level reduction. In addition, the peak-reduction effect was weaker when the floodplains were used only for flood storage than when flood discharge and storage were applied in combination. [Conclusion] Floodplains and polders are effective regulators for dealing with short-term excessive floods. In operation scheduling, it is necessary to balance the peak-reduction magnitude and the duration of action. Further studies should focus on the comprehensive effects of the combined operation of different types of floodplains and the adaptive management strategies.