Based on a vast amount of full and accurate data and investigations,this paper dis cussed the eutrophication state of Hulun Lake and found out key factors influencing eutrophica-tion and causes of relative strong self-purification function of the lake.Hulun Lake is the fifth largest lake in the semi-arid continental climate region of high latitude in China.It belongs to animal husbandry pollution.The nitrogen input to Hulun Lake mainly comes from polluted point sources(the rivers),while phosphorus input from area source is more than that from point sources.Through many years observatuion s,it is found out that the changes in nitrogen and phosphorus content s are not obvious,indicating the existence of a strong self-purification function of Hulun Lake.The self-purification function of Hulun Lake is related to its special geographical position.First,Hulun Lake is an alternately inflow-outflow lake linking tectonic lake and rivers with vast area and deep water.All of these con tributed to the self-purification function of the lake.Second,the activity of planktons in the lake is restrained by the higher latitude,semi-arid climate and low temperature.Third,the structure of the lake is special with over30mouths of springs and sufficient replenishment of underground water to the lake.Hulun Lake is a freshwater lake ecosystem consisting of a relative large water body and its surrounding wetlands.It not only places an important role in keeping biodiversity and enriching biological resources,but also functions obviously in adjusting and impounding flood s.Consider-ing its ecological and social benefits,Hulun Lake is indispensable to the development of commu-nity and industry,agriculture and animal husbandry.All of these showed that Hulun Lake bears an important position in the regional environmental conservation.