Abstract: Flexible mattress is a kind of beach protection structure,which is widely used in the waterway regulation projects in the middle and lower reaches of the Yangtze River.The beach stability has a great impact on the deformation and in-situ service condition of the flexible mattress.The local hydrodynamic environment near the beach is complex,and the failure model of the protected beaches is different from the failure model of the free slopes,so the failure model and its mechanism needs a further study.In this paper,a generalized physical model experiment is carried out to study the beach failure model and its mechanism under the protection of the flexible mattress.The study results show that:(1)According to the different mechanisms of the beach collapse,there are two types of beach collapse,traction type and sliding-covering type.The strain response value caused by the traction collapse is small,and the traction collapse occurs almost in the whole experiment time.The sliding-covering type collapse is an occasional failure phenomenon caused by the accumulation of boundary condition to a certain extent,and which leads to the large-scale sliding failure of beach and the large-scale subsidence deformation of the flexible mattress.(2)The sliding-covering collapse mechanism of the protected beach can be summarized as followed:under the protection of the flexible mattress,the potential slip surface of the beach is moving down by the water flow scouring.The angle between the potential slip surface and the horizontal plane is increasing with the increasing of scouring depth.The local stability of the beach is continuously reduced,which leads to the mechanical instability (F_s<1) on the potential slip surface and the widespread sliding-covering type failure inside the beach.