Abstract: With the successive construction and operation of water conservancy projects in the upper reaches of Changjiang River, significant changes have taken place in the distribution of annual water inflow and runoff in the main stream, which will inevitably have a huge impact on the operation of downstream reservoirs.Taking the reservoir groupin the upper reaches of Changjiang River as the research object, an operation simulation model for the upstream main and branch reservoirs and an optimal dispatch model for the joint power generation of cascade reservoirs from lower reaches of Jinsha River to Three Gorges Reservoir were established, and the impact of reservoir group regulation in the upper reaches on the power generation capacity of cascade reservoirs in lower reaches, from lower Jinsha River to Three Gorges Reservoir, in the medium and long-term scale were analyzed in detail.The research show that the regulation of upstream main and branch reservoirs would greatly change the distribution of annual water inflowof Xiluodu Reservoir and Three Gorges Reservoir, increase the proportion of inflow in dry season, and further improve the power generation capacity of cascade hydropower stations.Under the condition of wet year, the total power generations of cascade hydropower stations would increase by 1.22%,and the total abandoned water would decrease by 14.11%;in dry years, the total power generation would increase by 3.16%,and the total abandoned water would decrease by 15.67%.