The same-frequency area composition method was used to analyze the area composition of a design flood of the dam site of the Xiaonanhai Hydropower Station. The flood routing in the river reach of the dam site was simulated with the artificial neural network model, and the influence of the flood control operation of the upstream cascade reservoir group on the design flood of the Xiaonanhai Hydropower Station in different typical years was analyzed. The results show that the flood control operation of the upstream cascade reservoir group has a significant influence on the design flood of the Xiaonanhai Hydropower Station, especially on the reduction of rare floods. The floods that mainly come from the Jinshajiang River are greatly influenced by the flood control operation, while the floods that mainly come from the Minjiang River are not significantly influenced by the flood control operation.