Abstract: A 1D-2D coupled hydrodynamic model is developed by using a lateral linkage for numerical simulating of flood routine in flood protected zone with complex geometries and hydraulic interactions between river and flood protected zone. A Riemann problem based model coupling method is proposed to resolve the problem of no momentum exchange and the uncertainty of discharge coefficient from the traditional coupling method based on the weir equation. Besides, an adaptive time-stepping method is used for the 2D model to trace the time line of the 1D model. Numerical results show that the proposed model can effectively simulate the flood due to dyke-overtopping and dyke-break in the coupled system of river and flood protected zone, thus has bright application prospects.