There are numerous floodplains and polders along the middle and lower reaches of the Yangtze River, playing an important role in fflood discharge and storage, maintaining river stability, and protecting ecology. They are the homes that millions of people rely on for survival. Selecting typical floodplains and polders in the middle and lower reaches of the Yangtze River, including the Lower Jingjiang River reach and the Hukou to Datong River reach, the two-dimensional water flow mathematical models were established to calculate and analyze the flood storage effect under flood conditions after the operation of the Three Gorges and upstream reservoir groups in 1954. The focus was on studying the process of flood diversion flow after the use of floodplains and polders, the amplitude of flood level reduction at key control stations, and the decrease in maximum water level, the amplitude and duration of water level decrease, etc. The research results can provide reference and guidance for the graded optimization and operation of floodplains and polders in the middle and lower reaches of the Yangtze River, as well as flood control construction.