When large wood (LW) is transported along with debris flow, it usually tends to block in gully obstacles, resulting in sediment retention and gully siltation. However, these blockages are not stable and can cause serious erosion or bursting, resulting in catastrophic disaster. To better understand the influence of LW blockage on the debris flow process, the field experiments were carried out in the upper reaches of Fencha gully. The results indicate three types of LW blockage: non-blockage, unstable blockage, and stable blockage. The blockage probability increases with the length and content of LW, and the blockage probability of high sediment concentration debris flows is higher. Stable blockage can effectively retain sediment and slow down channel erosion, while unstable blockage exacerbates local erosion effects and peak discharge. When the relative length and content of LW are 1.0 and 0.04, and the sediment concentration is 0.424, due to the occurrence of unstable blockage, the flow amplification coefficient delta was 1.333. The unstable blockage is mainly caused by the uneven characteristics of LW interception, gully erosion, seepage deformation, and impact. The increase of blockage degree also prolongs the duration of debris flow. An identification method of blockage is established by analysis of duration extension effect and flow amplification effect. The results of this paper emphasize the hazards of LW blockage and gully erosion cause by woody debris flow and can provide reference for the mitigation of debris flow in forest mountainous areas.