Abstract：Large hydraulic projects play an important role in flood control， power generation， water supply， shipping， etc. While they change the natural hydrological regime of rivers and reduce the downstream flow， which greatly weakens the self-purification ability of river flow and poses a critical threat to the ecosystem functions and biological diversity of rivers. In this context， Shitoukoumen Reservoir and its downstream in the Yinma river basin was selected as the study area in this paper. In this study， the characteristics of COD and NH3-N and their changing trends in the Yinma River were first identified and how the present ecological discharge scheme of Shitoukoumen Reservoir improved the water environment quality of the downstream of Yinma river was investigated. Then， the ecological discharge of the reservoir was back-deduced under different water quality goals with the application of a hydrodynamic and water quality model. According to the simulations， a set of ecological discharge scheme of Shitoukoumen Reservoir was made to further improve the water environment quality of Yinma River and the feasibility of the scheme was demonstrated based on the water budget analysis of the reservoir. The results showed that volume of inferior V water reduced by 54.55% due to the implementation of the present ecological discharge scheme of Shitoukoumen reservoir. In order to achieve the water quality goal of the "14th Five-Year Plan" and further improve the water quality of Yinma river， more discharge should be released for ecological water supplement throughout a year except for October and December. Annual ecological discharge released from Shitoukoumen Reservoir will increase by 14.25 million m3. The proposed scheme can meet both the irrigation and water supply needs， and the impoundment plans of the reservoir. It can effectively alleviate the water pollution problems in the Yinma river and will greatly improve the water quality.