On July 5, 2021, a catastrophic debris flow disaster occured in Xiangjiao gully, Muli County, Sichuan province. This study analyzed the formation conditions, eruption process and eruption characteristics of the debris flow through field investigation and characteristic parameter calculation. Based on the results of on-site inspection, this debris flow was mainly caused by the combined effects of forest fires, short-term heavy rainfall, and channel topography. It was a post-fire debris flow caused by rainfall runoff erosion. The heavy rain caused powerful flood erosion on the slope surface, eroded the channel, led to collapse and landslides on both sides of the channel, and resulted in significant damming effects, which enlarged the scale of the debris flow. The forest fire area in Xiangjiao gully reached 74.61%, and the high-intensity burned area was 57.98%. The critical rainfall intensity for this debris flow was 77.84 mm/h, and the cumulative rainfall was 141.60 mm. According to the calculation results of characteristic parameters, the density of this debris flow was in the range of 1.83 ～1.93 g/cm3
, indicating it was a viscous debris flow. The flow velocity at the downstream outlet of the main channel was 7.22 m/s, and the peak flow rate was 759.08 m3
/s. Combined with the results of the rainfall-runoff method and the morphology investigation method, the recurrence interval of this debris flow was estimated to be once in a hundred years. Considering the development trend of debris flow, it is believed that there is still a possibility of large-scale debris flow in the basin. Therefore, prevention and control suggestions including slope reinforcement in the upstream, regulation in the middle reach, and drainage in the downstream are proposed.