Shuangchagou debris flow ditch is located in Pingwu County, Mianyang City, Sichuan Province. In 1982, a large debris flow broke out in the ditch, and in the summer of 2011 to 2013, a large number of debris flow deposits went down the ditch, blocking roads and rivers, and posing a threat to national Highway G247, villagers and Jiuzhaigou—Mianyang Expressway. However, its movement characteristics and impact characteristics on the piers of the Jiuzhaigou—Mianyang Expressway are still unclear.Methods: 〖WTBZ〗Based on the field investigation data and UAV aerial photography data, through physical model tests and numerical simulation methods, the characteristics of debris flow movement and the mechanism of disaster were analyzed, and the control measures were evaluated.Results: The flow velocity of the debris flow decreases with the increase of movement distance, and the impact force of debris flow is the largest at the bottom of the pier, and decreases with the increase of height and distance between pier and gully. Debris flow not only forms impact force on pier, but also piles up near pier and causes lateral pressure on pier. When the flood peak of debris flow passes through the bridge pier, the debris flow fluid will “climb” phenomenon, and the maximum pressure at the base of the pier will be 115. 0 kPa, and then the pressure will be distributed in the shape of “semi- elliptical ring” and transmitted vertically along the pier. The maximum principal stress of the pier is 0. 75 MPa, and the displacement at the top of the bridge will be about 0. 21 mm.Conclusions: Debris flow sources in the Shuangchagou Basin are mainly gully source, landslide and slope collapse source on both sides of gully and surface loose layer soil of cultivated land. Field parameter calculation shows that the debris flow is about 3. 99~7. 58 m/s, and the bulk density of debris flow is 1. 39~1. 67 g/cm3, which is a dilute debris flow. The numerical analysis shows that under the condition of a 10- year peak flood flow, the debris flow head will reach the Shuangchagou Bridge in 800~1000 s and may have an impact on the Shuangchagou Bridge. After 1200 s, the debris flow will reach the gully mouth, posing a threat to the residents of gully mouth, National Highway G247 and the Fujiang River.Based on the physical model test, the solid particles in the dilute debris flow are mainly deposited at the bottom, so the impact force at the bottom of the pier is the largest, which decreases rapidly with the increase of height. At the same time, the closer the pier is to the ditch, the greater the impact force. The accumulation characteristics of debris flow alluvium at gully mouth vary with the distance between pier and gully mouth. If the distance between pier and gully mouth is close, the higher alluvium accumulated near pier will cause a certain lateral pressure on pier. From the fluid—structure coupling numerical analysis, it can be seen that the debris flow fluid will “climb” when the flood peak passes through the pier. The maximum pressure of 115. 0 kPa will be generated when the pier base of the debris flow head is first contacted, and then the pressure is distributed in a “semi- elliptical ring” shape and transmitted vertically up along the pier. In order to prevent the impact of debris flow on the Jiuzhaigou—Mianyang Expressway pier, it is recommended to set up a blocking dam and a diversion channel upstream of the channel where the Expressway pier is located.