Based on water quality and water level data from 2008 to 2012, the spatial-temporal variation of water quality in the Poyang Lake was analyzed, and its response to water level change was quantified. The results show: (1) Water quality in the Poyang Lake was in deterioration since 2007, and was better in high water period because wetlands can degrade pollutants than in low water period. But sometimes the water quality went worse while the water level rose because the first flush contained many non-point source pollutants; (2) Significantly influenced by upstream pollutants from the Leanhe River and Xinjiang River, the spatial distribution of water quality shows an improving trend from the upper reach to the outlet in the main channel of the Poyang Lake, and also influenced by pollutants from counties nearby and internal pollutants from sediments during dredging; (3) A water quality and water level relationship formula was established, which shows the proportion of gradeⅠ～Ⅲ standard water in the Poyang Lake would increase by 6.2%, if the water level at the Xingzi station rise 1 m.