The impact liquefaction of sediments in the path of a landslide would reduce the friction of sliding surface, leading to increase in the speed and distance of the landslide. In this study, a self-designed impact-liquefaction test was used to explore the pressure development of soil under the effect of different impact energies. The microscopic mechanisms were unravelled by using Nuclear Magnetic Resonance (NMR) and Scanning Electron Microscope (SEM) methods. The results show that the soil at the middle depth shows higher pore water pressure than at the top and bottom of the layer. Furthermore, the variation of pore water pressure in these parts is relatively stable, and the peak pressure linearly increases with impact energy. The collapse of large pores in sandy silt is the primary reason for the generation of high pore water pressures in sandy silt. But when the soils are high-energy impacted, the mesopores and small-pores are compressed and blocked, which also stimulate the pore water pressure. The result also demonstrates weak drainage behaviour of sandy silt under rapid impacting. If the impact energy is sufficient, the pore water pressure in sandy silt suppresses the dissipation, causing quick liquefaction in the interior of the sandy silt.