Abstract: After decades of economic reform and booming industry development,a tremendouds amount of pollutant were discharged into the Dianchi lake and the lake is suffered from severe eutrophication. In order to investigate the pollution situation of the Dianchi lake,forty sampling locations were set at Waihai of the Dianchi Lake and pH,DO,transparency,total-P,dissolved total-P,dissolved-P,total-N,NH 3-N and Chla were analyzed monthly from Sep. 2001 to Aug. 2002. Four areas were defined geographical and each contains several sampling stations. They are the northwest area(sampling locations of 1-3),the north area(4-15),the middle area(16-25)and the south area(26-40). The results showed that the water quality of Waihai in Dianchi lake was worse than the standard of V level(Environmental quality standards for surface water),fell into the super-eutrophication state,only suitable for industry and agriculture use. Generally,the pollution status is as following,the northwest area was the worst,the north area was worse,the middle area was bad,then the south area. In addition,the spatial distribution of pollution was correlative with the industry and agriculture distributions. Although the concentrations of total-P,dissolved total-P,dissolved-P,total-N and Chla were the highest in the northwest area,and declined southwestward,a peak value of total-P was observed at the east part of the north area,and two peak values of dissolved-P were observed at the south area and the west part of the middle area. The contaminative degree was various greatly in different months, the concentrations scopes of total-N is from 1.11 to 10.27mg/L, total-P from 0.074 to 0.573mg/L, and Chla from 0.0136 to 0.327mg/L. It was observed that the pollution was worst in flood season (May to Sept.),while in winter and spring, the pollution was relatively better. The paper showed the pollution status and spatio-temporal distribution of pollution of Dianchi lake, which provided for the theoretic reference of divisional treatment Dianchi lake pollution. At last, the paper discussed the cause of eutrophication of the Dianchi lake, and gives some advice on the possible eutrophication prevention.