<p>With the increase of extreme climate in recent years, a large number of landslides and avalanches have frequently occurred. And the surge wave caused by avalanches often poses a serious threat to the stability of moraine dams. In this paper, the breaching mechanisms of moraine dams under the action of avalanche-induced surge waves were investigated based on flume tests. Combined with the surge wave erosion phenomenon and the variation characteristics of the water level in front of the dam, it was found that the occurrence of dam failure did not depend on a single surge wave or its maximum, but was closely related to the subsequent reflected waves repeatedly overtopping and eroding the dam. The more times and the larger height of the effective surge waves, the more intense the erosion on the dam, the more conducive to the occurrence of dam failure. And the evolution process of the dam failure under the action of surge waves can be divided into four stages: incipient motion of particles, formation of incision and gully, accelerated erosion, erosion attenuation. In addition, based on the classical equation of erosion rate, which is closely related to the erosion resistance and the shear stress, a criterion of overtopping failure for moraine dams under surge waves has been established, which considers the net overtopping height, effective erosion times and duration. When the total cumulative erosion depth leads to the effective dam height to be lower than the upstream water level, the dam can be considered unstable.</p>