Abstract: As the first large scale reservoir, Guanting Reservoir plays a key role on water supply for BeijingThe reservoir kept healthy in the first decade and water quality has deteriorated since 1972 graduallyIn the following three decades, the water body undertook series pollutions, and dropped out from the source region of drinking water in 1997 The evolution of water quality in Guanting Reservoir in the past three decades was summarized based on the previous studiesSources and characteristics of the water pollution in Guanting Reservoir were discussed by stagesAt the same time, variations of main pollutants were also discussed The results show that the evolution process of water quality in Guanting Reservoir could be roughly divided into four stagesThe feature of first stage was organic toxicants and heavy metals pollution during 1972～1975The following stage was from 1981 to 1992,in which organic pollution played a main roleThe water pollution in third stage was a kind of complex pollution, which added bacteria pollutants during 1992～1995From 1996 till now, nitrogen and phosphorous pollution were the key features in Guanting ReservoirContents of COD and main heavy metals increased slowly during the past three decades, while contents of phenol varied In addition, contents of nitrogen and phosphorous increased steadily and significantly.

Key words: Evolution of water quality, Guanting reservoir, Water pollution