Abstract: The severity of a landslide-debris flow is determined by such factors as the speed, deposition morphology and impact force of the sliding body. Differences in the lithological characteristics and structural distribution of the sliding source area lead to differences in the particle order distribution and particle size of the sliding body. During the movement, the collision, friction, and jumping between the particles affect the degree of hazard of the landslide-debris flow. Based on the physical model test, the dimensional discrete element software PFC^3D is used to explore the influence of the particle order distribution and particle size on the speed, stacking morphology and impact force. The results show that the average velocity of particles in the debris flow is affected by both the particle size and the initial particle order in the slip source region, and the initial particle order has a greater effect on the average velocity of particles. The particle size has a greater effect on the particle order arrangement in the thickness direction, while the particle order distribution in the slip source area has a greater effect on the deposition morphology. Under the effect of particle-size segregation, the particle size becomes the main factor controlling the peak impact force and the slip source. The particle sequence distribution in the slip source area determines the accumulation morphology, which controls the impact force of the debris flow in the quasi-static accumulation stage.