Through thorough analysis of the ecological security of lakes and reservoirs, which is the foundation of people' s living and regional sustainable development, the related mechanism between water system and social activities is elaborated. Then on the basis of pressure of human activities, the water quality and the service functions, the ecological security situations and trends of Dianchi Lake at time scale from 1999 to 2007 are analyzed. The results indicate that contamination gross control effectively relieves the pressure of human activities to some extent. But the water quality isn't obviously improved, and the grade of ecological security is not optimistic according to increasing index from 2005.