The Dongting lakeshore plain city has complex flood problem due to its flat terrain， which is normally located below the high water level for flood control in the lake area, so it is necessary to conduct research in areas where floods can not be completely eliminated. The paper chooses Yueyang City as a case study to make the numerical simulation of hydrodynamics with MIKE21 FM model. In the flood simulation，the design level of the dike design level and the highest water level in the history are selected as the water level input in the model， the breach location is set, and the submergence, flood flow, and flood inflow at different periods of the flood evolution are simulated. The result shows that the MIKE21 FM model has high accuracy and real simulation effect， and has certain value in the flood simulation of lakeshore plain city. The research on the flooding simulation method of the plains in the lake area has direct significance to flood prevention and relief work in Yueyang City and in the lakeshore plain cities of China.