Abstract: In previous experiments, insufficient reservoir capacities have led to significant deviations between the presented breach process and the actual situations. To address this issue, large-scale physical experiments were conducted on the breach of landslide dams with three different reservoir capacities to study the effects of barrier lake storage capacity on the overtopping breach process. The results show the following: ① Under different reservoir capacity conditions, the typical stages of the dam overtopping breach process are consistent, and the breach discharge hydrographs also show relatively similar patterns. However, there are differences in peak magnitudes and peak occurrence times. When the reservoir capacity is doubled, the peak discharge increases by 118.8%, and when tripled, the peak discharge increases by 271.0%. ②Under constant impounded water conditions, when the upstream reservoir capacity is larger, the water level rises more slowly and the rapid initial rise phase of the breach flood occurs later. Moreover, when the erosive action of the water flow on the dam is stronger, the lateral widening rate of the breach during the rapid development stage is faster. In addition, a larger peak discharge at the breach is associated with more drastic variations in the discharge, as well as with sharper and narrower hydrograph curves. ③ The breach plane exhibits a trumpet-shaped form. While the upstream reservoir capacity has little effect on the location of the breach's narrowest section, it significantly impacts the final breach width: the larger the reservoir capacity, the greater the breach width.