Abstract: The construction and operation of the cascade reservoirs in the Upper Yangtze River have significantly altered inflow and sedimentation of Three Gorges Reservoir （TGR）, which makes the original design floods and flood limit water level （145 m） fail to meet the updated situation. Since the spatiotemporal distribution of storm-flood is uneven owing to many large-scale reservoirs in the Upper Yangtze River, the current flood regional composition methods are unable to directly derive the design flood hydrograph in such complex cascade reservoir system. The mixture copula function was adapted to construct joint distribution of annual maximum flood data series and typical flood events simultaneously, the most likely and unfavorable flood regional composition methods were modified to estimate design floods in the TGR operation period, and the corresponding flood control water level were derived. The results show that the mixture copula function can improve the practicality and stability of the flood regional composition methods in complex cascade reservoir system; due to the regulation of upstream reservoirs, the TGR design floods have been reduced about 15%—20%; under the conditions of unchanged flood prevention standards and flood protection high water level, the corresponding flood control water level （155.5 m） is estimated through flood routing calculation. This research will provide theoretical and technical support for the modification of TGR regulation rules and joint operation of hydraulic projects in the Upper Yangtze River.