[Objective] By analyzing the formation mechanism and dynamic characteristics of the debris flow disaster, this paper aims to provide a scientific reference for prevention and mitigation research of the future disaster.[Methods] By field investigation and analysis.[Results] The disaster formation conditions of the Rangjia gully, in Anhua County, Yiyang City, Hu'nan Province, including topography, formation lithology and hydrology, are all prone to the formation of large-scale debris flow. Due to the continuous heavy rainfall, a large-scale landslide was formed and blocked the gully channel, but then the landslide-dam failed and formed outburst debris flow with the peak flow up to 190.65 m³/s.[Conclusion] The most striking feature of this debris flow is the conversion of landslide to debris flow. However, two manmade check dam along the downstream of the channel were all destroyed by outburst flow from the upstream, which magnified the peak flow.