Typhoon rainstorm is an important rainfall type that induces geological disasters in the coastal areas of South China. Affected by the landfall of typhoon “Utor”, the typhoon rain directly triggered the extra-large slag-type debris flow disaster in Wushikeng Gully in Xingning, which caused huge material sources and great potential harm. At present, the research on the slag debris flow in typhoon and rainstorm is still at the initial stage, mainly focusing on its formation mechanism, disaster characteristics and other aspects. There are problems such as narrow research scope, single method and few achievements. Therefore, it is of great significance to study the formation mechanism and dynamic characteristics of the slag debris flow in typhoon and rainstorm for the disaster mechanism and hazard evaluation of such debris flow. Methods: We collected, sorted and analyzed the geological environment background. The Wushikeng Valley in Xingning is a typical typhoon-rainstorm-landslide-debris flow disaster chain area, which is a high incidence area of debris flow. The volume and physical and mechanical parameters of the material source were explored through measurement, drilling, density and particle grading in-situ tests and laboratory tests. The channel deposits accounted for 81. 9% of the total material source. In combination with the material composition and deformation and damage characteristics of the slag pile, the stability of the slag pile was calculated and evaluated by the slice method. We collected and analyzed the rainfall data during the typhoon period. The rainfall and rainfall intensity exceeded the critical rainfall index for triggering debris flow in the region. Calculated the dynamic characteristic parameters of debris flow such as flow velocity, flow rate and fluid impact force and their change rules, further analyzed the characteristics of debris flow movement and accumulation process, and quantitatively analyze and evaluate the debris flow activity trend from the aspects of debris flow activity duration, rainstorm intensity index and risk assessment. Results: The formation mechanism of slag type debris flow in typhoon rainstorm: its unique watershed topographic characteristics are the basic factors for the formation of debris flow, the rich slag type material source mainly composed of channel deposits is the basic condition, the high intensity typhoon rainstorm is the direct trigger factor, and the joint action of mine mining, random stacking of slag and other factors has expanded the disaster scale. The formation process and dynamic characteristics are summarized as follows: early heavy rainfall-landslide debris flow-slag debris flow-typhoon rainstorm-channel blocked and highed-collapse and scale enlargement. The comprehensive debris flow has a strong rainstorm index R=6. 41, occurrence probability of 0. 2~0. 8 and hazard degree Hof 0. 6260. The debris flow is in the youth stage and belongs to highly dangerous debris flow. Typhoon and rainstorm often occur in this basin, and it is very likely that large-scale debris flow will break out again. Its harm and impact range is larger. Therefore, the prevention and control of debris flow should be studied. Conclusions: This paper takes the debris flow in Wushikeng Gully in Xingning as an example, takes the geological environment of debris flow as the research background, studies the formation conditions and mechanism of debris flow, and studies the dynamic process characteristics of the slag debris flow in typhoon rainstorm through theoretical calculation. Typhoons and rainstorms occur from time to time in the basin, and it is very likely that large-scale debris flows will erupt again, therefore research on the prevention and control of debris flow should be carried out, which provides a reference for the disaster mechanism of such debris flow and the prevention and mitigation work in similar disasters.