Abstract: A destructive debris flow broke out in Meilong gully, Danba County, on June 17, 2020, discharging almost 427000 cubic meters of solid material, triggering a disaster chain consisting of debris flow, debris-flow dam, outburst flood, and landslide, forcing more than 5100 households and 21200 people to transfer and causing direct economic losses of 800 million yuan. The authors analyzed the causes and mechanism of the Meilong gully debris flow based on on-site investigation, aerial photography, and remote sensing interpretation, and elucidated the influence of material source, rainfall and terrain on the formation of debris flow. The results are shown as follows: (1) The Meilong gully debris flow was formed by the interaction of accumulated rainfall and short-term heavy rainfall. (2) The source of the debris flow in Meilonggou is in Dashibao ditch, which was caused by the collapse of the ditch bank. (3) The cascade blocking effect caused by the recharge of material and the three massive landslides named Dongfeng Shed, Meilong village, and Dayi village further amplifies the quantity of the debris flow. A large amount of solid material rushed out of the ditch mouth as a result. (4) The disaster chain consisted of debris-flow dam, outburst flood, and landslide at the mouth of the ditch, which further enhanced the risk of the debris flow. (5) The abundance of material in the Meilong ditch and lower rain threshold will tend to trigger destructive debris flow in the rain season. It is recommended to take comprehensive prevention and control measures in time.