Erhai Lake known as a "bright pearl" on Yunnan Plateau the second largest plateau lake in Yunnan is located in the central part of Dali prefecture. In 1996 blue agal bloom was first observed in Erhai Lake which showed that Erhai Lake had undergone overwhelming eutrophication.The possible causes of eutrophication in Erhai Lake were analysed in detail. Investigations revealed that the blue algal bloom was on the mid-trophic status indicating that nutrient accumulation was not the only factor. Some environmental and biological changes were observed especially in 1990s due much to the human activities interruption. Finally, some countermeasures for eutrophication control are put forward.