Abstract: For the large-scale flood control system of the middle Yangtze River-Dongting Lake, featuring by complex flow conditions, data scarcity, and a limited lead time, a hierarchical and modular flow simulation model has been proposed, combining together 1-D with 2-D flow models, hydraulic with hydrologic methods, numerical algorithms with supplementary information. A finite-volume method for calculating 2-D unsteady flows on an unstructured mesh is used for the lake of a complex configuration, to preserve mass balance. A conservative explicit difference scheme is applied to simulate branched and looped network unsteady flows, to avoidtedious matrixcal culations caused by using implicit schemes, and facilitate the 2-D model and various flow connections. This hybrid unsteady flow model has been utilized to flood routing for the system connected through sub-lakes, river networks, and flood-plains.