Abstract: The soil parameters of landslide dams typically exhibit significant spatial variability. Investigating their impact on dam failure processes is of great scientific importance for emergency response and disaster mitigation. In this study, landslide dam models with different grain-size accumulation structures were designed and subjected to overtopping breach tests. The breach duration, breach evolution, and flow characteristics at different breach stages were systematically analyzed, and the specific influence of vertical grain-size accumulation structure on the breach process was discussed. The results showed that, due to differences in the properties of surface materials, landslide dams with different grain-size accumulation structures exhibit significant differences in failure duration, breach evolution, and peak discharge during the initial breach stage. When the surface material had low cohesion, lateral slope instability in the spillway manifested as planar shear failure; when the cohesion was higher, overturning failure occurred. Coarse particles in the surface material hindered the erosion and incision process during the acceleration stage. Under similar average particle sizes, landslide dams with a normal gradation structure (finer particles on top and coarser particles at the bottom) exhibited a more significant flooding threat during failure. Differences in the properties of the bottom materials mainly influenced the post-failure channel morphology. Specifically, landslide dams with an inverse gradation structure (coarser particles at the bottom and finer particles on top) tended to form channels with relatively gentle slopes after failure.