Abstract: Debris flow as non-Newtonian fluid has a large yield stress and moved unsteadily. Many models have been established to study the impact of erosion on the unsteady motion, but most of the models were studied the role of the bottom erosion and dam break on the debris flow motion. Less researches studied the role of gully bank erosion on the unsteady dynamic of debris flow. This paper analyzed the role of bank erosion on the debris flow dynamics through flume experiments by bank erosion-dominant and bottom erosion models. The experiments showed that the normal stress and pore water pressure of debris flow increased unsteadily along with the increase of the height of the debris flow head, but the bank erosion made the fluctuation more obvious. The mechanical equation demonstrated that bank erosion enhanced the increase of the frictional resistance. However, the greater velocity difference between the flow body and flow head made the additional gradient of the flow head greater under the bank erosion-dominant condition, so the bank erosion increased the average velocity of debris flow. The dynamic balance between head resistance and power lead to the unsteady motion of debris flow.