As urban systems become increasingly complex, urban rainstorms and flood disasters exhibit characteristics of complexity and cascading effects. A single actor is insufficient to address such complex disasters, making the enhancement of collaborative governance capacity for cascading disasters imperative. A two-layer network model of collaborative governance of cascading disasters is proposed from the “risk-capacity” coupling perspective, depicting both risk propagation pathways and the structure of organizational response capabilities. By establishing a prevention-and-control mapping relation between the organizational collaboration layer and the disaster cascading layer, the model evaluates the governance effectiveness under different collaboration strategy scenarios. Using the “23·7” extreme rainstorm event in Beijing as an empirical case, this paper finds that rainstorm-induced flooding readily evolves into urban systemic risks, with infrastructure damage serving as a critical pathway for risk propagation. The collaboration network exhibits a non-equilibrium capacity structure characterized by “government-led, enterprise-mediated, and socially-marginalized” roles. Institutionalized organizations remain the main actors in disaster governance and can achieve approximately 68% risk reduction. Furthermore, a multi-actor collaborative governance mechanism involving social organizations can further reduce systemic risk, whereas fragmented actions by social organizations may lead to resource misallocation and negatively impact disaster mitigation outcomes.