Abstract: The temporary block-breach phenomena tend to occurrence when debris flows run throng bayonet segment, accompanying with instant scale-amplified effect. By the way of field investigations as well as indoor flume tests, the influence factors and formation mechanisms of the block-breach effect were studied herein. Results showed that the flow amplification caused by the blocking effect of debris flow or the flow attenuation caused by the accumulation of solid materials in the bayonet segments, were related to the geometric conditions of the bayonet, the debris flow fluid and trench features, and whether there was additional loose material . The existence of bayonet was mostly a positive factor for the flow attenuation of debris flow, and whether there was the addition of loose materials was the key factor for the blocking effect of debris flow in bayonet segments. This study provided theoretical significance and application prospects for understanding the motional and deposited regularities of debris flow in where micro-topography abruptly changed, and improving the technical regulations of natural hazard prevention and mitigation.