A large number of towns in the western mountainous areas are within the hazard range of disaster chains such as landslides and debris flows, making the safety and disaster risk of mountain towns a key focus. The landslide-debris flow disaster chain in Lijie Gully, Lijie Town, Zhouqu County, Gansu Province was taken as a case study. Based on field investigations and remote sensing analysis, surface deformation was analyzed using interferometric synthetic aperture radar (InSAR) technology to determine the source of the disaster chain. The RAMMS dynamic model was then employed for the process and risk analysis of debris flows, followed by vulnerability and risk assessment. The research results indicate these as follows. The main subsidence areas within the basin are located at the edge and top of the ancient landslide area above Lijie Gully on the North Mountain, with the main cause of deformation being the seasonal freezing and thawing of soil. Based on the deformation results and remote sensing image analysis, the RAMMS dynamic model is used to conduct risk analysis of the Lijie Gully debris flow under three rainfall frequencies of 1%, 2%, and 5%. The comprehensive evaluation results show that the high-risk area accounts for 70.51%, largely distributed in the loose accumulation surface of the North Mountain landslide, the gully, both banks of the gully, and the accumulation fan at the gully mouth. Based on the risk and vulnerability assessment results, the high-risk area of the Lijie Gully debris flow accounts for 13.10%, mainly distributed in the gully mouth and along both sides of the gully where buildings are located. The large volume of loose accumulation above Lijie Gully is still in a continuous process of creeping-deformation-sliding, providing a large amount of material source for debris flows. The risk zoning results under different rainfall frequencies provide a reference for the disaster reduction of urban debris flows.