Abstract: The soil parameters of landslide dams typically exhibit significant spatial variability. Investigating their impact on the dam failure process is of great scientific importance for emergency response and disaster reduction. In this study, a flume experiment was conducted with landslide dam models designed using different grain-size distribution structures. A series of overtopping failure tests were carried out to systematically analyze the failure duration, breach evolution, and peak discharge characteristics at various failure stages, with a focus on the impact of vertical grain-size distribution on the failure process. The results show that, due to the differences in the properties of surface materials, landslide dams with different grain-size distribution structures exhibit significant differences in failure duration, breach evolution, and peak discharge at the initial failure stage. When the surface material cohesion is low, the lateral slope instability of the spillway manifests as planar shear failure; when the cohesion is higher, it results in overturning failure. The coarse particles in the surface material help hindered the erosion and incision process during the acceleration stage. Under similar average particle sizes, landslide dams with a normal gradation structure (i.e., finer particles on top and coarser particles at the bottom) exhibit a more significant flooding threat during failure. The differences in the properties of the underlying materials mainly affect the post-failure channel morphology. Specifically, landslide dams with an inverse gradation structure (i.e., coarser particles at the bottom and finer particles at the top) tend to result in channels with relatively gentle slopes after failure.