Abstract: In order to comprehensively examine the water quality of Caohai Lake and offshore areas of Dianchi lake; eleven main areas,including: Caohai Center,Broken Bridge,Caohai Dam,Donfeng Dam,Middle Section of Gray Bay,Luojiaying,Guanyin Mountain,Haikou,Dian lake in southern,Baiyukou and Gudu Terminal,were targeted for water quality investigation The main indicators affecting the water quality of Dian Lake are ammonia nitrogen,total nitrogen,permanganate index,five-day oxygen demand and total phosphorous were selected for the trend analysis of water quality change. The results showed that water quality of Dianchi Lake is tend to be class IV,while the total nitrogen content pollution was at critical level. A slightly change and lower concentrations have been observed other indicators.