Analysis of water quality and evaluations show that the Erhai Lake is in a mesotrophic state. According to eval-uations on some annual average state and on all lake nutrition types at some period of recent years, it is feasible that Erhai Lake now is assessed to the transition type between mesotrophic and eutrophic state. Compared with historical data, from the 1980s the pace of eutrophication was a little fast and the nutrients elements concentration increased gradually. Factors that result the changes are: (1) increasing of contaminants within the catchment; (2) lake being at low water level for a long time, and (3) unreasonable exploring of natural resources.