Monitoring data in 2001 indicate that inner lake in Miyun Reservoir has reached eutrophication because of pollution of net-cage fishculture.The main water body of reservoir belongs to mesotrophication and tends to be eutrophication.The water body of inner lake exchange substance with main water body of reservoir through permeation and spread.When the water lever is over 150m, the waters between water bodies are linked together.Inner lake pollutes reservoir region seriously.Inner lake is a pollution source in Miyun Reservoir.For stabilizing water quality of Miyun Reservoir, it is necessary to stop activity of net-cage fishculture in inner lake.