In order to further verify the safety degree of the longitudinal reinforced rockfill dam against the flood overtopping damage compared with the conventional rockfill dam dam, this paper analyzes the erosion of downstream rockfill caused by the flood overtopping based on relevant experiments, during which the downstream material is washed away forming a scour hole that is partially exposed to the air at the downstream side. Besides, the safe operating mechanism of the reinforced dam is also obtained from the experiments. The advantage of this new dam is changing the conventional overtopping failure mode to the progressive scour hole development mode, therefore it can delay the dam breaking, and provide additional time for the repair and evacuation. Calculations show that, whether the reinforced body will fail depends on the scour depth and its ultimate stress distribution.