As the water source of the middle route of South-to-North Water Transfer Project, Danjiangkou Reservoir boasts an ever-increasingly prominent function of water supply and other benefits after the dam was heightened. Nevertheless, the decline in rainfall and the regulation and storage of cascade reservoirs in the upper reaches of Hanjiang River have greatly reduced the transferable water resources of Danjiangkou Reservoir. In order to give full play to the benefit of the reservoir, we calculated and analyzed the influences of the starting and regulating water level, the uncertainty of incoming water and the limited water level of flood season on the benefit of Danjiangkou Reservoir from three aspects: water supply satisfaction rate, average annual electricity generation and average annual waste water level. Results manifested that 1) water level at the incipience of regulation has a great influence on the water supply benefit in dry year when the frequency of incoming water is more than 90%. When water level is raised from 150 m to 160 m, the rate of meeting water supply demand can be increased from 44.38% to 93.42%. When the frequency of incoming water is less than 75.7%, water level mainly affects power generation. The higher the water level at the incipience of regulation is, the higher the annual power generation is, and the smaller the annual waste water is. When water level is raised from 150 m to 170 m, power generation could increase by at least 1 billion kW·h. 2) The higher the frequency of incoming water, the smaller the rate of meeting water supply demand, and the lower the average annual power generation. When incoming water is abundant, water abandon and water supply shortage both exist in the mean time. 3) The influence of flood limit water level in summer on the benefit of Danjiangkou Reservoir is more significant than that of flood limit water level in autumn; in a three-year regulation period with at least two years of low water, flood limited water level exerts a most obvious influence on abandoned water quantity. 4) Water level at the incipience of regulation is the major factor that affects the operation benefit of the reservoir. Except from continuous dry years, the water supply demand of different level years can be met if the water level at the incipience of regulation is 162 m.