Abstract: Floods caused by dike break and overtop can be simulated with a dynamically coupling model of 1D and 2D hydraulic model. Generally, the traditional 1D and 2D flood routing models are operated separately. Moreover, the traditional model would take a great amount of time to simulate, and the given results are not accurate. In this paper, a 1D-2D dynamic coupling model for the flood routing is developed through time synchronization and corresponding of space nodes. According to dry depth and wet depth theory, the traditional flood routing model is optimized, which saves much time for simulation. An example of Guduiwei flood detention basin is shown for rerifying the 1D-2D dynamic coupling model. Historical floods are simulated with the 1D-2D dynamic coupling model, which has verified the correctness of the model. Based on the results, a flood risk chart is established with standards. The results show that the 1D-2D dynamic coupling model is reasonable in simulating dike break-induced floods with much time saved.