The transition of hyper-concentrated flow and debris flow after earthquake is the key problem involved in debris flow engineering control.Based on the analysis of debris flow in Moutuo Gully,this paperstudies the importance of source supplement and micro-topography in the transition of hyper-concentratedflow and debris flow.The terrain and landform are analyzed firstly;secondly,the density is used to analyzethe transition character between hyper-concentrated flow and turbulent debris flow;and finally,landscapestep-pool effect method and investigation method are applied to study the transition process and mechanisms.The result shows that(1)the initiation model of debris flows in Motuo Gully includes surface erosion,gullyerosion,blocking,outburst and debris flow;(2)the formation of microtopography(step-pool)is resultedfrom waste slag and collapse or landslide;(3)the transition of hyper-concentrated flow and debris flow isclosely related with the solid volume in one surge.