Abstract: Within the hierarchical decision structure of reservoir pre-impoundment operations, the parameter equifinality of hydropower generation leads to non-uniqueness of optimal solutions, i.e., the “ill-posedness” of solving reservoir operation optimization problems. Under such circumstances, the realization of operation benefits is affected by whether the reservoir operator selects the refill plan in favor of flood safety, implying that not only competitive relationship but also cooperative potential exists between flood control and water conservation. In light of this, a cooperation incentive（CI） model based on the lower-level satisfaction is developed to provide a mechanism to promote the water conservation department’s cooperation with the flood control department and enhance the reservoir operation benefit. Based on the framework of ill-posed bilevel programming, regarding the actual decision characteristics, the model described the nonlinear correlation between the cooperation willingness and the expected benefit of the water conservation department given certain flood control rule, so that the probability of selecting the refill plan in favor of flood safety can be derived. The CI model is solved by using multi-swarm evolutionary particle swarm optimization algorithms. Quantitative indicators are proposed to evaluate the Pareto efficiency loss and overall goal achievement of the reservoir operation optimization under cooperation. In the Three Gorges Reservoir pre-impoundment case study, the results are compared with those of the optimistic, pessimistic, and partial cooperation models. Results show that the CI mechanism motivates the benefit concession of the flood control department to increase hydropower generation and encourage the water conservation department’s choice in favor of flood safety. Further, the efficiency loss in operation decisions due to competitive gaming process can be more prominently reduced. Findings also indicate that nonlinear satisfaction-expected benefit relationship can better describe the practical decision making in reservoir operation.