Dam-break is a type of catastrophic process with instantaneous and huge destructive effects, which can cause sudden scouring and silting of the riverbed and even instability and collapse of the river bank in the downstream of the river. In this study, a two-dimensional numerical model based on the shape of the natural channel and the actual layout of the existing spur dike group was constructed using the MIKE21 FM software. The main objectives of this study are to simulate the downstream water and sediment transport process after the sudden failure of the first permeable spur dike, and then to analyze the changes in riverbed shear stress and the alluvial characteristics of the riverbed near the spur dikes. The results show that after the instantaneous failure of the first spur dike, the flood evolution is influenced by the downstream spur dikes and changes its process. However, the dam-break has little effect on the sediment transport capacity of the main channel, but significantly affects the incipient and settling motions of sediment in the area between the spur dikes. Furthermore, since the body of the first spur dike and the head of the second spur dike are the most vulnerable areas, a composite configuration structure of permeable and impermeable dikes is recommended for the spur dike group layout to improve its stability during operation, which is impermeable dikes for the first and second spur dikes and permeable dikes for the rest of the dike group.