To address the challenge of coordinating reservoir group operations for human benefits, disaster mitigation, and ecological protection in river basins, this study introduces an externality quantification mechanism and a three-stage game framework, proposing an ecological regulation theory for reservoir groups from the perspective of the human-fish game. First, it reveals that reservoir operation and fish survival are not inherently a zero-sum game, clarifying that while maximizing human development needs, ecological negative externalities can be minimized and positive externalities maximized to achieve a synergistic win-win outcome for both humans and fish. On this basis, a three-stage game framework for ecological regulation based on externalities is proposed. Through hierarchical optimization-multi-objective foundational equilibrium, minimization of negative externalities, and maximization of positive externalities-a three-layer optimal equilibrium solution for complex game problems is achieved. Finally, key technologies for ecological regulation of reservoir groups based on the three-stage game are presented, encompassing externality assessment, refined simulation, and multi-objective scheduling. This theory aims to provide an theoretical paradigm and systematic practical pathway for reconciling human development needs with fish survival requirements, facilitating the harmonization of ecological priorities and human-centered development.