Abstract: Yangcheng Lake is the third largest freshwater lake on the Taihu Plain.Based on the monthly monitoring data of water quality of Yangcheng Lake from 2019 to 2023 and the field survey data of water environment in flood seasons in 2023, this paper analyzed the temporal and spatial variation of water quality in flood seasons.The causes of pollution was also analyzed from the perspectives of exogenous input, endogenous release and pollution reduction of aquatic vegetation.The results showed that the water quality of the Lake in the flood seasons from 2019 to 2023 was mainly characterized by Class IV.The change of water quality index concentration in a year demonstrated a certain periodicity, and the water pollution degree in the flood season was increased year-on-year.Total phosphorus was the main over-standard factor of water quality.The total phosphorus in the lake was mainly composed of dissolved inorganic phosphorus.From the perspective of spatial distribution, the water quality of East Lake was better than that of West Lake and Middle Lake.The water quality of non-restoration aquaculture area and aquatic vegetation restoration area was better than that of enclosure aquaculture area.The pollutant concentration at the estuary point had a significant impact on the water quality of Yangcheng Lake.The contents of permanganate index, total phosphorus, total nitrogen and other indicators at the estuary point were significantly higher than those in the lake area, which was the key factor affecting the water quality of the Lake.The results showed that the phosphorus content in sediments of the purse seine culture area was higher.During flood seasons, the increase in temperature resulted a decrease in the dissolved oxygen of water, and consequently a high risk of release of the total phosphorus from the sediments.In addition, aquatic vegetation planting may have a strong correlation with transparency improvement, permanganate index reduction and chlorophyll a reduction.The reduction of phosphorus by aquatic vegetation may be mainly related to the content of total dissolved phosphorus.