The major rivers across the world have become artificial regulation systems controlled by cascade reservoirs. River fish, as an important part of freshwater ecosystems, are under threat of population degradation and biodiversity loss in the process of development and utilization of river water energy resources. The ecological operation of the reservoir is eco-environmental protection to alleviate the negative impact on ecosystems, which artificially creates hydrological and hydrodynamic conditions to meet the requirement of fish during key life periods. However, based on ecological operation practical experience, there are circumstances where downstream flow conditions are inconsistent with the fish requirement in local periods affected by different operation conditions of reservoirs. That means the ecological operation still needs improvement. This paper takes the Three Gorges Dam, the world's largest power station, and the downstream Gezhouba Dam, as a case study, builds a joint operation model of cascade reservoirs, simulates and analyzes the feasibility of the cascade reservoirs to improve the spawning conditions of fish in the downstream river reaches under joint operation and discusses the optimal strategy. In most cases, the target of continuous flow rising for hourly timescale can be accomplished only by the hourly exquisite operation of Gezhouba reservoir, which is required for the four major Chinese carps spawning downstream of Gezhouba Dam. What's more, the hydropower generation output can be improved to a certain extent, thus achieving win-win progress of ecology and economics. The results of this paper show that Gezhouba reservoir can adjust the daily discharge process of the Three Gorges Reservoir by the re-regulation during a specific period of ecological operation, which may function as a useful reference for the optimization practice of ecological operation of cascade reservoirs.