Abstract: The debris flow directly transformed from a landslide usually has a longer mobilization distance, wider impact, and more serious hazards than the landslide itself. However, there is limited research on the transformation mechanism and its influencing factors. On October 20, 2020, a small debris flow occurred in Hujiaya, located on the left bank of the Bailong River in Zhouqu County, Gansu Province. It flowed onto S313 and threatened the adjacent village. Based on field investigations, laboratory geotechnical tests, remote sensing interpretation, and rainfall analysis, this paper explores the geo-environmental conditions, mobilization, and accumulation characteristics of the Hujiaya debris flow. The main characteristics, processes, influencing factors, and formation mechanisms of a shallow landslide directly transforming into debris flow are studied. The future trend of Hujiaya debris flow is also discussed. It reveals that: (1)The loose materials, accumulated in the narrow-long channel of the upper reaches of the Hujiaya gully, were mobilized due to a long-duration rainfall of up to 19 days and a total of 98.5 mm, resulting in a slow-moving landslide. The damage to the slope structure caused by previous regional strong earthquakes also contributed to the landslide occurrence. (2)The back segment of the landslide and the sheared-out part of the front segment of the landslide were directly transformed into viscous debris flow during their movements. However, the middle segment of the landslide continued to slide. (3)The transformation process is greatly influenced by the soil saturation that sheared out at the front segment of the landslide, the steepness and height drop of the gully, and the ground runoff. (4)The rear edge of the Hujiaya gully may be damaged due to the progressive traction of the adjacent Xieliupo landslide. Additionally, there are residual landslide masses and limestone avalanche masses in the channel upstream and the residual landslide mass on the right side of the outlet, which could mobilize due to future similar rainfall events and strong earthquakes, transforming into debris flow again and threatening the downward Hujiaya village and highway S313.