The immigrants from Houziyan hydropower station are resettled on the accumulation fan of Jiangkou gully debris flow. Based on field investigation on the formation conditions and motion characteristics of the debris flow, the hazard scope of debris flow occurred in 1958 with a frequency of 50-year return period is obtained, and the motion parameters of debris flow are determined with the rain-flood method. The fluid level distribution and velocity field are simulated, and the hazard scope and the velocity field of Jiangkou gully debris flow are obtained by using CFX software, based on Bingham rheological model. The simulation results show that the average velocity of a 50-year return period debris flow passed through the accumulation fan is 5.76 m/s, while the maximum velocity is 13.59 m/s. The hazard zone obtained by the numerical simulation is larger than that occurred in 1958, since the deposit of early debris flow filled up the original ground, especially the ground near the channel, making the debris flow flood and expand more easily toward both the flanks of the channel. This research provides a new approach to designing prevention project and delineation of hazard zone of debris flow.