Abstract: Under the background of global climate change, mountainous areas in western China show an obvious warming and wetting trend, and the risk of flash flood and debris flow disaster chains triggered by extreme heavy rainfall continues to rise. Such disaster chains experience multi-stage flow regime evolution, and traditional monitoring methods fail to capture their entire transformation process, resulting in inadequate prevention and control capacity. Taking the flash flood and debris flow disaster chain that occurred in the Xinglong River Basin of Yuzhong County on August 7, 2025 as the research object, this paper analyzes its formation causes from the perspectives of extreme heavy rainfall, multi-source initiation on slopes and in channels, temporary channel blockage and breaching, and human activities based on field investigation and remote sensing interpretation. The total volume of loose materials supplied by landslides and channel deposits is calculated to be 1.14×106 m³. Abundant loose materials sharply increased the magnitude and solid concentration of flash floods, which further evolved into debris flows, and temporary channel blockage and breaching pushed the peak discharge of debris flows to approximately 2 732 m³/s. On this basis, seismic ground motion signals generated during the event are adopted to reveal the mutual transformation characteristics of the disaster in the form of "flash flood-debris flow-flash flood" and illustrate the spatiotemporal evolution of multiple flow regimes. Targeting this type of disaster chain, this paper proposes to establish an integrated early identification system for material sources combining space, airborne, ground and in-situ monitoring, build a monitoring and early warning model based on seismic ground motion, carry out risk assessment of disaster chains and inspect human activities in channels. The research findings can help improve the prevention and control capacity of flash flood and debris flow disaster chains induced by heavy rainfall in mountainous areas of western China.