［Objective］Post-fire debris flow is a specific type of debris flow with universal common characteristics， originating from large forest fires. It is found that there is a "roasted pigskin effect" in post-fire debris flows，which is primarily attributed to the fire-calcined effect of wildfires on the underlying surface vegetation and rock-soil media.［Methods］Numerical simulation was used to analyze the fire-calcination effect on the underlying surface and reveal the temperature conduction process and changes of temperature stress on the underlying surface following the wildfire disaster.［Results］The depth of influence of wildfire on the underlying surface could reach 3 m，and the temperature stress could reach 405 MPa，which could easily cause thermal cracking and decomposition of the underlying rock and soil . The concept of the post-fire material sources was proposed，and a structure model of the post-fire material sources was established，which included two parts，namely，the volcanic ash layer and the fire-calcined layer，with the former being the primary source of the initial stage of post-fire debris flow，and the latter being the primary source of the mid-and late-stage of the post-fire debris flow. Based on the post-fire material sources，the mechanism of post-fire debris flow formation was explored，i.e.，the stage of surface-washing evolution for ash material sources and the stage of gully erosion evolution for the fire-calcined layer，the former appeared in about two years after the wildfire，while the latter spanned a much longer time scale，potentially reaching millennium to ten thousand years. From the perspective of "roasted pigskin effect" of post-fire debris flow，strategies for prevention and control of post-fire debris flow were explored.［Conclusion］The research results have positive significance for studying the formation mechanism and targeted management of post-fire debris flow.