In the context of water shortages, environmental flow is the key to alleviating the negative environmental impact of reservoir operation. Environmentally, the suitable stream flow for the survival and reproduction of aquatic organisms is a varying range, although most of studies now give it a fixed value, which leads to unreasonable resource allocations. In this study, we proposed a fuzzy representation of environmental flow by using the fuzzy theory and the ecological hydraulic radius. Furthermore, we used the Three Gorges-Gezhouba cascade reservoirs as a study case and Four Major Chinese Carps as indicator species. In addition, a multi-objective operation optimization model was established, which solved by the Evolver Palisade software, and a multi-objective risk analysis method was proposed based on the design reliability and risk rate of various benefit operations. The results show that: (1) Based on the environmental flow membership function, flow ranges suitable for the aquatic organism survival and reproduction at specific locations can be determined to help guide reservoir discharge. (2) Taking environmental flow membership as an optimization objective rather than a constraint is conducive to formulating environmentally friendly reservoir operation schemes and make more rational use of water resources. (3) The multi-objective risk analysis can avoid the one-sidedness of a single objective and analyze the risks brought by various benefit operations. Ecological demand has long been a factor considered when formulating reservoir operation schemes. Following the environmentally friendly operation scheme is helpful to protect the environment and maximize the overall benefits of reservoirs.