Abstract: A generalized Lab-test in a flume was performed to simulate a typical riverbank collapse called a nest-shaped flow slide in the middle-lower reaches of the Yangtze River. To explore failure mechanical mechanisms, the characteristics and variation in water flow structure near the caving pond during the collapse formation and development were studied using by fine survey data of the flow field. Results show that the flow around the pond can be divided into three zones:the main flow, the eddy flow and the circumfluence zones, where the flow structure changed continuously with the collapse development. In the main flow zone outside the pond mouth there is basically an open bend channel current with obvious circulation characteristics. In the eddy flow zone appearing a strip-shape there are many vortices varying in size, shape, position and strength that generally move down with the main flow rapidly, and breaks down or decomposes continuously. At the upper and lower ends of the pond mouth, the current clearly exhibits the three-dimensional characteristics, with strong and violent water surface fluctuation. In the circumfluence zone there is a typical cavity flow, and as the collapse develops, the circumfluence expands, its center moves down, its shape changes from elliptical to circular, and its strength shows an increase-decrease-stabilization trance.