Abstract: On March 30, 2020, a forest fire broke out in Jingjiu Township, Xichang City, and the vegetation in Xiangshui gully watershed was burned in a large area by forest fire. In the rainy season of the same year, debris flow broke out in many gullies in the Xiangshui gully watershed, of which 1#, 2#, 3# gullies were adjacent to residential houses and cultivated land, and the impact of the debris flow was relatively serious. Through field investigation, remote sensing interpretation, and outdoor test, taking Xiangshui gully 1#, 2#, 3# gully as the research object, the differences of permeability characteristics, slope erosion, and gully erosion under different forest fire intensities are analyzed, so as to reveal the disaster mechanism of post-fire debris flow of Xiangshui gully. The results show that forest fire is an important inducement for the outbreak of debris flow, and the rainfall threshold of debris flow after the fire will be significantly reduced. Forest fire disturbance leads to the decrease of the permeability coefficient of slope soil in varying degrees. The more serious the forest fire intensity is, the smaller the permeability coefficient is, and the rainfall is transformed into slope runoff in a larger proportion to participate in slope erosion. With the increase of rainfall times, the depth of slope soil erosion in light, moderate and severe fire areas increased. The difference of erosion depth between moderate and severe forest fire intensity is small, and it is significantly higher than that in light areas, indicating that when the forest fire intensity reaches moderate, the slope soil will be eroded to a greater extent. For the gully with similar topographic conditions, the more serious the forest fire intensity is, the stronger the erosion capacity of debris flow is, which is finally reflected in the greater the number of landslide bodies on both sides of the gully and the greater the width and depth of the gully.