Abstract: The study on the mechanism of the channel avulsion in the tail reach of the Yellow River Estuary（YRE） involves issues such as characteristics of flow and outflow, water and sediment volume, and river bed boundary conditions. On the basis of proving the recent swing characteristics of the river channel in the tail reach of the YRE, this paper reveals the mechanism of the channel avulsion in the tail reach of the YRE and the temporal and spatial changes of the river bed boundary conditions, and analyzes their impact on the outflow of the river based on the measured topographic data. The results show that, the coefficient of fluvial facies is the critical influencing factor to determine the avulsion position of the river channel in the tail reach of the YRE. The variation characteristics of the horizontal gradient of the floodplain and the change of the position of the deep river line determine the swing direction of the river channel.