In 2019, the catastrophic Zhaiban slope debris flow (ZSDF) crushed an open road tunnel in Ganluo County, Sichuan Province, China, causing seven deaths and interrupting traffic flow for 16 days. We analyzed the characteristics of the ZSDF disaster by undertaking a comprehensive on-site investigation, evaluating the internal and external dynamic characteristics, and conducting a numerical simulation. The formation mechanism of typical slope debris flows in the arid valley area of the Hengduan Mountain was revealed, and the material foundations for generating the ZSDF were identified as internal dynamic effects on the slope (including thrust faults and folds), historical earthquake occurrences, and external dynamic effects from an extreme dry and wet cycle (with a low frequency of once in 55 years). Soil failure occurred in a hollow catchment region at the top of the gully (covering an area of 74,084 m(2)) under a heavy rainfall of 33.4 mm/h, and large boulders (average radius of 2.43 m) were pulled along the gully and down the steep slope gradient, thereby providing the large amount of kinetic energy required for the ZSDF. Large boulders and soil masses remain in gullies on slopes within the study area, providing the high possibility of future slope debris flows under heavy rainfall. The results also suggest the use of large-gradient flumes on top of open tunnels in the arid valley area of the Hengduan Mountains to provide optimum protection against slope debris flow.