Abstract: Post-fire debris flow is a kind of debris flow that is closely related to forest fire. The post-fire debris flow in the Sejiao Ravine of the Sanyanlong township in Jiulong county of Sichuan is takan as a typical example, and the developmental characteristics and the formation mechanism of the post-fire debris flow is systematically examined by means of on-site investigation, remote sensing interpretation, indoor and outdoor experiments. The results show that the ash layer, especially the surface soil, of which the physical, hydrological and mechanical properties were changed after being baked by forest fire, constitutes the source which is different from the conventional debris flow. The average coefficient of permeability of the disturbed soil in the burned area is 4.31×10-5 m/s, while the average coefficient of permeability of the soil in the unburned area is 7.52×10-5 m/s. Different permeability lead to the difference between slope-runoff and the change in the catchment conditions in the gully, and reduce the rainfall threshold of the debris flow, so as to affect the trigger mechanism of the debris flow in the burned area. It indicates that the development characteristics and disaster mechanism of the post-fire debris flow are totally different from the conventional debris flow.