Abstract: Beijng City is short of fresh water seriously.Water resources of every person is about 300m3.Miyun Reservoir is the chief drinking water resources in Beijing,which influenced people’s lives and economic development greatly. In order to know the trophic state and limitative factors on Miyun Reservoir,water quality of the reservoir and the valley’s economic situation were investigated.Eight sits(fixed position by GPS)and twelve determinable items were selected in the reservoir and they were water holding capacity,water depth,water temperature,transparency,pH,TN,TP,CODmn,BOD 5,DO,Chlorophyll a and phytoplankton.Two investigations were finished,one in May 2001(dry season)and the other in September 2001(season for rain plenty). The research results indicates that nutrition characteristic of Miyun Reservoir belongs to responding type of phytoplankton.Community structure of phytoplankton belongs to green algae-diatom type. Total nitrogen in water is 0.86mg/L and total phosphorus is 0.022mg/L.The cells density of phytoplankton is 405.23×104/L.TSI M of four targets(Tr、TP、CODmn、chla) are from35.8.to 49.6,water body of the lake has reached mesotrophy.But the TN?TP and cells density of phytoplankton are on the increase,water body tend to be of eutrophication. Primary factors are water and soil erosion in upper reaches of two rivers and pollution of the water body.Miyun Reservoir is principal drinking headwater for Beijing.It is very essential to protect ecological environment of the valley and to harness pollution.