The destruction of mountain landform by forest fire often leads to the outbreak of catastrophic debris flow. In March 2020, a huge forest fire broke out in Xiangjiao Gully, seriously damaging the mountain landform and leading to the outbreak of a catastrophic debris flow on July 5th, 2021. To mitigate such disasters more scientifically, this paper analyzes the disaster characteristics of the "7.05" catastrophic debris flow and studies the cause mechanism and mitigation countermeasures. The results show that the debris flow density ranges from 1.831 g/cm(3) to 1.930 g/cm(3), belonging to viscous debris flow. The velocity of each section point is between 7.22 and 9.78 m/s, with huge kinetic energy. Forest fire, torrential rainfall, and gully terrain conditions combined to cause the outbreak of the debris flow. The rainfall intensity of the maximum rainfall point reached 77.84 mm, corresponding to a recurrence period of more than 200 years. The forest fire severely damaged the landform, and the high-intensity burned area reached 59.97%, which led to a surge in loose materials and wood debris. Combined with steep terrain, channel erosion and the blocking-bursting effect are prominent, leading to the scale amplification of debris flow. The peak flow of the upstream section D-1 and the downstream section D-6 are 308.21 m(3)/s and 759.08 m(3)/s, with the scale expanded by 2.46 times. To mitigate the harm of Xiangjiao Gully debris flow, this paper deduced the determination method of the optimal hydraulic section of a Rectangle-V-shaped drainage canal under the limited conditions. The calculated characteristic parameter of the section is m=11.75, h(2) = 0.60 m.