To address the problem of flood control optimization on the mainstream and tributaries, a joint optimal operation model for a mixed-connected reservoir group was established with the objective of risk degree balance. Based on the large-scale system decomposition-coordination theory, a three-layer hierarchical structure consisting of the system coordination layer, the tributary subsystem coordination layer, and the single-reservoir optimal operation layer was designed, and corresponding efficient solution methods were developed. A case study of joint operation of the flood control system in the Hanjiang River Basin demonstrates that the proposed method achieves good risk balance among reservoirs, with the coefficient of risk degree variation of four reservoirs below 6% in all scenarios. After operation, the peak reduction rate of Danjiangkou Reservoir reaches 37.68% to 67.36%, and the error of flow rate simulation at Huangzhuang Section is controlled within 3%. The proposed method has high computational efficiency, with the calculation time for all scenarios within six seconds. Keywords: mixed-connected reservoir group; flood control operation; risk degree balance; large-scale system decomposition-coordination; piecewise heuristic optimization algorithm; the Hanjiang River Basin 〖FL