[Objective] The origin, formation process and characteristics of the "8·20" debris flow at Xiazhuang gully, Wenchuan, Sichuan Province in 2019 was explored in order to provide a scientific reference for the prevention and early warning of debris flow disasters in the region.[Methods] Based on field surveys and remote sensing images, debris flow source supply conditions, debris flow formation, erosion transport and river blockage process were analyzed. And FLO-2D model was used to analyze the extent of Xiazhuang debris flow blockage under different rainfall intensities.[Results] After the earthquake, part of the debris flow in the debris flow basin continued to erode the source of the channel, resulted in the accumulation of gully material source. Runoff formed by short-duration heavy rainfall continuously eroded the gully material source and formed a channel-initiated debris flow. A large amount of debris from the "8·20" debris flow in Xiazhuang gully was transported out of the ditch, and formed a dammed body with a length of 280 m along the river and a maximum width of 110 m across the river. The blocking mode of the river was full blockage of the dam. Under the frequency of 5-year rainfall, the material source would stay in the channel without blocking the river. Under the frequency of 20-year rainfall, the debris flow basically would block the Zagunao river and form a blocking phenomenon. Under the frequency of rainfall once in 50 years, the debris flow would completely block the Zagunao river and form a dammed lake.[Conclusion] Xiazhuang gully and Zagunao river intersect vertically, and the diversion dam of a hydropower station is built at the mouth of the gully, which is easy to form the phenomenon of dam blocking. During the flood season, when the rainfall intensity is high, debris flows may still cause river blockage. In the future, the prevention and control of debris flows need to strengthen flood prevention and early warning and other disaster reduction measures.