Abstract: Debris flows carrying large boulders often cause more severe damage to buildings and structures than conventional debris flows. The "August 3" extreme large debris flow in Ridi Gully carried large boulders, destroyed an expressway Bridge, buried houses, and blocked the Kangding River. Based on field investigation and existing research results on large boulders in debris flows, this paper analyzes the disaster-causing characteristics of the debris flow. It also explores the distribution, impact characteristics and causes of large boulders in combination with UAV aerial survey images, and puts forward corresponding prevention and control suggestions. The impact force of debris flow slurry in Ridi Gully is 102.39 kN, and the maximum impact force of large boulders is 30,638.54 kN, which can be more than 300 times that of slurry. Collapses and landslides are extremely developed in the Ridi Gully Basin, with 16 landslides and 157 collapses. The debris flow discharge is large, and the peak debris flow discharge at the broken bridge can reach 1,600 m³/s. The distribution density of large boulders is positively correlated with the development degree of collapses and landslides in the basin. Frequent earthquakes intensify the development of collapses and landslides and promote the formation of large boulders. New mountain fissures exist in the Ridi Gully Basin, which is under great threat of mountain collapse and landslide.