In the context of the Yangtze River Conservation and international wetland protection, this study investigated the characteristics and causes of eco-environmental changes in Lake Honghu. Four field surveys were conducted from 2019 to 2023, focusing on the lake's water quality, sediment parameters, submerged plants coverage and phytoplankton biomass. Results showed that: (1) During the survey period, the average water secchi disk (SD) of Lake Honghu decreased year by year, and SD showed a decreasing trend from the center to the shores during the construction period of the returning polders to lakes; (2) The nitrogen and phosphorus nutrient pollution in the lake remained severe. Although the concentration of total nitrogen (W-TN) and ammonia nitrogen in the water body showed an overall decrease, the average total phosphorus concentration in the water (W-TP) increased from (0.06±0.04) mg/L to (0.19±0.06) mg/L. Total phosphorus in the sediment (S-TP) increased by 32%, while S-TN and S-TP level decreased during the polders returning period, and increased afterward. Spatially, W-TN and W-TP levels showed a decreasing trend from the northwestern inlet of the four main canal to the southeast; (3) The degradation of Lake Honghu ecosystems was observed. Submerged plants such as Potamogeton crispus L., Myriophyllum spicatum L. and Ceratophyllum demersum L. were present in April 2019, but had almost disappeared by October 2020 in the open-water area. The average concentration of chlorophyll-a increased from (13.92±7.74) μg/L to (62.64±19.52) μg/L, dominated by filamentous cyanobacteria. Principal component analysis showed that TN and TP as consistent primary drivers of the aquatic environment changes, and SD emerging as a key factor in May 2023. The contribution of nitrogen and phosphorus nutrients from upstream inflow, hydrodynamic processes following polders returning causing pollutant migration within the area of polders, and wind and wave disturbances affecting the sediment were key factors driving changes in the aquatic environment of Lake Honghu.