The contradiction between the development and population growth in flood storage and detention areas of the dikes has become increasingly prominent. It is thus of great value to conduct quantitative risk assessment of flood disaster for these areas, and a crucial step for risk assessment of flood disaster is to estimate the consequences induced by dike-break. To accurately estimate the losses of life, economy and ecological environment caused by dike-break, this paper developed an approach for modeling evolution process of flood based on MIKE21 FM model. The methods for evaluating the losses based on the inundated data from the evolution process of flood in the dike storage and detention area were presented. The flood storage and detention area of Kangshan Dike in Poyang Lake was taken as a typical case to simulate the evolution process of flood under the historical highest water level. The actual inundated data including inundated area, water depth and flow velocity of this project was obtained; based on which, the disaster grades in the disaster area were defined. The losses of life, economy and ecological environment in the flood storage and detention areas were estimated by combining the flood evolution with the losses calculation. The research results can lay a foundation for quantitative assessment of the dike-break risk and provide an effective reference for formulating flood prevention and rescue decision-making schemes for the flood storage and detention areas.