Abstract: During power transmission, dry landslide debris flow usually causes severe dynamic erosion when interacting with loose, water-bearing basement materials on slopes or within channels. This interaction leads to a significant increase in the scale of debris flow, posing a serious threat to infrastructure and human lives. Simultaneously, interstitial fluid is entrained into the bottom of the debris flow with the solid particles. The exchange of materials makes the flow change from single-phase to solid-liquid two-phase state, critically influencing the rheological and mechanical behavior of the debris flow.However, few studies investigated the fluidization mechanism of debris flow. This study employs smoothed particle hydrodynamics-discrete element-finite element (SPH-DEM-FEM) coupling theory, combined with large-scale physical model tests, to investigate the complex dynamic interactions between dry debris flows and substrates under varying moisture conditions. The results show that the contact between the leading edge of the debris flow and the basement is mainly characterized by punching failure and ploughing, and the contact surface is dominated by shear abrasion. With the increases of basement stress and pore water pressure, the debris flow is mixed with water-bearing material, and gradually presents the fluidization characteristics. Under the impact loading of debris flow, the base stress in the eroded area exhibits an “advanced fluctuation” phenomenon. The stress of the leading edge shows a significant increase due to the impact of debris flow. The base stress in the middle of the eroded area shows a parabolic curve with a slight amplitude and a long duration due to the leap of particles. The base stress of the trailing edge presents a curve with a peak value lower than that of the leading edge. With the change of substrate material from dry condition to unsaturated condition, the shear stress, and moisture content of the contact surface show a positive correlation trend. The erosion rate of the substrate, the impact distance of the debris flow, and the final accumulation thickness display a parabolic correlation with the moisture content. The results provide the understanding of the fluidization mechanism in debris flows and provide effective scientific insights for the study of similar mechanisms.