Le'an River, located in the northeast of Jiangxi Province (southeast China), is a tributary of China's largest freshwater lake, Poyang Lake, which drains directly into the Yangtze River. The watershed covers an area of app. 8000 km², with an average annual precipitation between 1687-1870 mm. There is no huge settlement in the watershed, of which forest and farmland accounts for 95% in the area. According to the historical data, the water quality is much worse in wet seasons than in dry ones, so it is believed that mainly diffuse pollution, especially caused by agriculture, influences the water quality. Based on data from 17 monitoring stations within Le'an River watershed from 2009 to 2011, major influence factors, ingredients of diffuse pollution, and the extent of pollution have been analyzed. The main conclusions of this paper are:1) NO₃^--N has the highest content and most important influences on water quality among the 3 physical forms of nitrogen. TP has a comparably minor influence on water quality. The whole watershed can be divided into 4 districts according to the cluster analysis. The water quality remains to be type Ⅱ in the upstream area with at least 90% forest coverage. Eutrophication increases from upstream to downstream along with an increasing share of farmland, illuminating that anthropogenic induced land use change is the major factor of diffuse pollution. 2) The amount and intensity of precipitation play a significant role on nutrient concentration. Rainfall could increase surface runoff instantly, introducing abundant pollutant into the water body and leading to rapidly increasing in nutrient concentration. 3) Except in major flood period of the year, the maximum volume of water body could dilute nutrients greatly. In a year with regular or high rainfall, water quality appears fine with ample water volume, but it turns bad in a year or season with less water. Moreover, with soaring frequency of extreme meteorological events caused by global climate change and the construction of large-scale hydro-power project in the Yangzi River basin, Lake Poyang's water shortage will become regular, the water volume will decrease continuously, as well as the lake's self-purification capacity, therefore positive and effective countermeasures should be taken to improve the watershed sustainability. 4) Although the present water quantity of Lake Poyang is good, there are still pressures caused by economic development and climate change, which is threatening the watershed sustainability. Stakeholders and decision-makers shall work more effectively on environment conservation in the upstream area in particular with high attention to reasonable landuse plan, improving soil quality, and enhancing watershed environment capacity.