The Luanshibao (LSB) landslide is a prehistory long-runout landslide located in southeast Tibet. Its maximum horizontal sliding distance exceeds 3900 metres. The landslide exhibits an apparent friction angle of 11.7°, while the apparent friction angle within the depositional area is only 0.9°. Due to its high mobility and potential threat to the highway and railway lines constructed in its deposits, the long-runout movement mechanism of this landslide remains a research hotspots. We conducted a detailed field investigation of the LSB landslide and discovered that the fluvial sandy sediments in the slope-front basin area played a crucial role in the long-runout movement. Therefore, this sandy samples were collected to conducted a series of ring-shear tests, studying its mechanical behaviour in response to the impact of the landslide mass. Based on the experimental results, 3D simulation on the entire process of the LSB landslide movement was carried out by MPM making use of its advantages in simulating the large deformation. The study mainly focused on analysing the impact process of the sliding mass onto the front-slope sediments. The numerical simulation results indicate that the impact process of the landslide may exhibit in two distinct stress paths (normal stress strengthening or normal stress weakening) based on its dynamic coefficient (the ratio of impacting pressure to pressure caused by static weight), which is determined by the mass of sliding material, the sliding velocity, and the duration of the impact. When the dynamic coefficient exceeds 1.7, the loading can disrupt the sedimentary structure of the front-slope sandy layer and lead to a complete loss of strength, even though the sediment has not reached a liquefied state. This impact process would entrain the sandy layer into the new sliding zone, resulting in the long-runout movement and the final flat-spread shaped deposition.