Abstract: According to the survey results in 2000 and 2001,the nutritive peculiarity of water body in Miyun Reservoir belonged to responding type of phytoplankton.The structure of phytoplankton community was of Chlorophyta(51.6%)-Bacillariophyta(29^2%) type.The proportions of Cyanophyta,Pyrrophyta,Chrysophyta,Cryptophyta and Xanthophyta are 6.1%,6.04%,3.27%,2.83% and 0.95% respectively.Among the phytoplankton,the main were middle-nutrition indicator species.But there were some eutrophicational indicator species.In 2000,the cell density of phytoplankton was 376.2×10 4L-1.The phenomenon of vertical distribution of phytoplankton community was very obvious.General tropic state index suggested that water body of Miyun Reservoir already belonged to middle nutrition and there was the tendency of eutrophication.So we should find measures to control its development.