Abstract: Revealing the spatio-temporal evolution mechanism of flooding disaster chains in mountainous cities, analyzing the cascading transmission effects of such chains, and quantifying the risk level of disaster chains can provide technical support for chain-breaking disaster mitigation in mountainous regions. Taking historical flooding disasters and the typical “23.7” extreme rainstorm event in Mentougou District, Beijing as an example, this study establishes a risk assessment model for flooding disaster chains in mountainous regions, clarifies the spatio-temporal evolution relationships of these chains, evaluates the risk magnitude of the disaster chains, distinguishes the robustness of flooding disaster chains and the disaster-bearing system, and proposes engineering measures based on node protection and critical path blocking, as well as non-engineering measures based on disaster monitoring, simulation, and emergency evacuation and rescue. The results show that in the flooding disaster chain of Mentougou District, urban flooding (C11) and traffic disruption (E13) are key nodes, and the edge centered on economic property loss (G13) is the vulnerable path of the disaster chain system and the key outlet for risk outbreak; the comprehensive risk value of the disaster chain is 23.909, with node risk contributing 80.4%. When disaster prevention measures are implemented, the robustness of the disaster-bearing system is enhanced, and the disaster chain risk decreases, showing a negative correlation between the two. Using complex network models to clarify the complexity of flooding disaster chains in mountainous cities and to identify key nodes and vulnerable edges helps in formulating targeted disaster prevention and control strategies and measures.