Climate change and human activities have altered the hydrological patterns of Lake Poyang, leading to lower dry season water levels and extended dry periods. Additionally, extreme events such as the floods of 2020 and the prolonged droughts of 2022 and 2023 have occurred. These changes have significantly impacted the growth and reproduction of floodplain plants. This study investigated the dominant floodplain plant, Carex cinerascens, examining five water level recession timing scenarios: 30 days earlier, 15 days earlier, normal, 15 days later, and 30 days later. The study observed the plant's growth and reproductive responses. The results showed: (1) Water recession timing significantly affected the maximum height of Carex cinerascens. Earlier recession increased plant height, while later recession reduced it. The impact on aboveground biomass varied seasonally. In autumn, delayed recession significantly decreased aboveground biomass, while in spring, there was no significant effect. Both spring and autumn underground biomass decreased with delayed recession, affecting biomass distribution between seasons. (2) Water level recession timing also significantly influenced vegetative reproduction in the autumn growth stage. Earlier recession increased the number of tillers and the quantity and length of spacers, while delayed recession inhibited tillering and reduced spacers' quantity and length. Both earlier and delayed recession affected the number of flowering stems and inflorescences in spring, but the impact was smaller with earlier recession compared to delayed recession. These findings provide a theoretical basis for the conservation and management of dominant wetland plants under changing hydrological conditions.