Abstract: Abstract The Donghekou landslide-debris flow is a typical rapid long run-out landslide triggered by the great Wenchuan earthquake. It started from the elevation of 1300m and caused 780 deaths, with a total long run-out of 2400m. Field investigations show that, during the sliding process from start-up to stop, this landslide-debris flow experienced the following five dynamical stages, such as the starting-up stage, the gravity speed-up stage, the enclosure air cushion effect stage, the impact reentry stage and the long run-out sliding accumulation stage. Finally, it reached the Honghuadi village, which is located at the left bank of the Xiasihe River, and the dammed lakes occurred. By analyzing the geological setting and feature of this landslide-debris flow, we can draw a conclusion that the initiation of Donghekou landslide-debris flow is mainly affected by the fault crush belt, the local convex landform and the phyllite and carbon slate with weak mechanical property; for the rapid and long run-out sliding of this landslide-debris flow, the water containing condition of the slope debris on sliding route is an important factor besides the advantage landform.