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Located in the Xigu District of Lanzhou City, Gansu Province, the Siergou watershed has historically experienced large-scale debris flows that have caused significant casualties and property damage. Based on the field survey and remote sensing interpretation, we studied the characteristics of the material source and influencing factors of the debris flows in Siergou through existing literature and indoor tests. We used the FLO-2D software to simulate and analyze the risk of debris flows. The results show that Siergou is dominated by viscous debris flows, which exhibit low-frequency activity and are currently in recession. There are abundant material sources in the Siergou watershed, which can be classified into four types: slope-type, landslide-type, ditch-type, and manmade-type, among which the landslide-type and ditch-type source control the outbreak scale of debris flow. The volume of a one-time flush-out mainly depends on the development degree of a landslide when the landslide occurs. The more developed the landslide is, the larger the one-time flush-out volume is and the larger the scale of the debris flow is. Under critical rainfall conditions, debris flows will break out in Siergou and deposit on the circulation area, forming medium-high risk areas, which seriously threaten the safe operation of the infrastructure in the gully, such as the Lanzhou-Xining high-speed railway and the beltways of the Lanzhou City. When extreme heavy rainfall is encountered, larger-scale debris flows will break out in Siergou. Therefore, further study of the risk of debris flows under extreme weather conditions is necessary to provide a geological basis for debris flow prevention and mitigation in this region. [ABSTRACT FROM AUTHOR]