iyun Reservoir is the largest one in the North China.The principles for determining the operation rules of Miyun Reservoir in flood season were laid down twenty years ago.In the present view,such a largedesigned flood storage has reduced the reservoir storage capacity.By deepgoing analysis of the hydrological characteristics of the Chaobaihe River and taking account of the effect of the three new-built reservoirs on the floodpeak discharges in the downstream watershed,this paper proposes some new schemes.Each of the schemes canobviously increase the benefits of Miyun Reservoir.