Abstract: The bank erosion of sediment through the exchange of momentum and energy transfer within the debris flow affects the unstable motion of the debris flow. Many models have been established to study the impact of erosion on debris flow initiation and motion, but most of the models were based on bed erosion. This paper analyzed the process of unstable motion of debris flows through an experimental flume to contrast bank erosion-dominated conditions and bed erosion-only conditions. The experiments showed that bank erosion enhanced the formation and propagation of debris flows. The height and velocity of the debris flows fluctuated along the path under both conditions. However, bank erosion made the motion fluctuation more obvious. Bank erosion usually occurred at the body of the debris flow and the particles moved more rapidly in the body than in the head, resulting in rapid growth of the head by the contribution of particles from the body. The height and additional gradient of the bank erosion-dominated debris flows were higher than those of the bed erosion-only debris flows. So the velocity of debris flow for bank erosion-dominated conditions was higher than that for bed erosion-only conditions.