In order to study the influence of the sea-entering channel phase Ⅱ project on the discharge capacity of the Hongze Lake and its surrounding detention basin, a 1-D and 2-D coupled hydrodynamic numerical model is established, based on the generalization of river network and topographic treatment. Four years' historical flood data of 1991, 2003, 2006 and 2007 are used for the calibration and verification of the coupled model. Based on the present working condition, planning condition and different flood magnitudes, five plans are designed for comparison and numerical simulation. The flooded area and the population and GDP affected by the flood are analyzed. The results show that the sea-entering channel phase Ⅱ project will decrease the water level of the Hongze Lake efficiently, and increase the flood control standard of the Hongze Lake and the activation standard of the surrounding detention basin. The sea-entering channel phase Ⅱ project will reduce the population and regional GDP influenced by the same flood magnitude, and it will provide opportunities for the flood control and safety construction in the Hongze Lake area and for the stable development of the surrounding detention basin.