Abstract: Landslide-debris flow is a sudden geological disaster, which has the characteristics of high speed and strong destructiveness. When landslide-debris flow occurs, the grain composition has a great influence on the movement speed of landslide-debris flow. In this paper, 14 different gradation conditions of single particle size group, three particle size group and whole particle size group were set to explore the effect of grain composition on the movement speed of landslide-debris flow. The fractal dimension D was introduced to quantify the grain composition, and the relationship between the maximum velocity and grain composition was obtained. The results showed that in single particle size condition, when the particle size developed from small to large, the friction form between the particle flow and the bottom surface changed from sliding friction to rolling friction, the friction energy consumption of particles decreased, and the velocity of debris fluid increased. Under the condition of mixed particle size, the landslide-debris flows had obvious stratification during the movement process, and the initial gradation can determine the thickness and structure of the cushion. With the decrease of the fractal dimension D and the fine particle content, the cushion thickness during the motion also decreased synchronously, leading to the increasing velocity of the debris flow