Rainfall debris flow is a multi-coupling process including rainfall–flood–sediment transport–bed load movement. The disaster risk of Baiyangcheng Gully debris flow is analyzed based on GDEM–GAVA software. First, through historical disaster data, the accurate mechanical parameters of the channel material source are back-analyzed. Then, based on the parameters obtained from the inversion, the critical rainfall duration of the debris flow under different rainfall intensities is analyzed. The calculation results showed that the saturated cohesive force of the debris flow in Baiyangcheng Gully is 0.5~1 kPa, and the saturated internal friction angle is 5°~10°; when the rainfall intensity is 65 mm/h, the formation time of the debris flow in the channel is 3.6 h; when the rainfall intensity is 78 mm/h, the formation time of the debris flow in the channel is 2.8 h; when the rainfall intensity is 88 mm/h, the formation time of the debris flow in the channel is 2.3 h. The analysis results of the above-mentioned debris flow disaster risk can provide the necessary basis for the scientific early warning of the Baiyangcheng Gully debris flow.