Abstract: At around 21 pm on June 26, 2020, a massive debris flow broke out in Damawu Village and Dabaozi Village, Yihai Town. This massive debris flow caused heavy casualties and property losses. After the disaster, the causes of debris flow were analyzed by collecting local basic information, combining it with field investigation and field testing. The debris flow was caused by the early earthquake and human activities, which led to abundant loose solid materials in the basin, and the loose solid materials were not dredged and treated in time. The slope of the upper and middle reaches of the basin is large and the terrain is steep, which provides good natural conditions for the development of debris flow. On the evening of June 26, heavy rain suddenly fell, which surpassed the rainfall threshold and caused the flood of the Anning River to wash away the dam and triggered the massive debris flow. The debris flow disaster gives us a major warning: we should strengthen the publicity of debris flow disaster prevention and mitigation knowledge, guide local residents to choose reasonable sites to carry out engineering activities and dredge the river course in time, strengthen the monitoring and early warning of mountain disasters, and improve the construction of group detection and prevention systems.