Abstract: Algal bloom is characterized with large scale and a long lasting time in Dianchi Lake.The main species forming algal bloom is Microcystis and M.aeruginosa is the absolutely dominant species.Algal bloom occurs from April to November every year and usually distributes in Haigen,Hui Bay and the south part of Caohai,sometimes even all the lake.the maximum chlorophyll concentration of surface water reaches 5 000 mg/m³.High content of N and P in the water of the lake is the main reason responsible for the algal bloom formation while suitable natural conditions also contribute to the process.