Forest fires cause serious damage to mountain landforms and trigger frequent post-fire debris flows. Although post-fire debris flow exhibits time evolution, the key factors controlling its evolution remain unclear. A detailed field investigation, rainfall data collection and remote sensing analysis were conducted to study the debris flow events following the "3.08" forest fire in Xiangjiao gully. The destructive effect of forest fires, the control factors and inherent evolution mechanism of post-fire debris flow were explored. The results highlight that the great disturbance of forest fires to the hydrological response and material source supply conditions promote the outbreak of debris flows. In the rapid response stage of fire, the internal driving force of debris flow evolution is the self-healing of hydrological response characteristics of the basin, including material depletion, particle coarsening and vegetation restoration. In the long-term impact stage, the evolution of debris flows is mainly controlled by factors such as a decrease in root-soil strength caused by root rot, multi-stage gully bank landslide activity, and blockage of woody debris. A conceptual model for the evolution of post-fire debris flows is proposed based on the above evolution characteristic analysis. In particularly, this study emphasizes the catastrophic effect of woody debris during the evolution of post-fire debris flows. The research results provide scientific basis for long-term debris flow risk assessment and mitigation design in recently burnt areas.