Abstract: Small reservoirs in hilly areas serve as crucial water conservancy facilities, playing a significant role in flood control, irrigation, and other aspects. However, in the event of a dam breach, severe hazards to downstream cities can occur. This paper takes Guangyuan City as an example and employs numerical simulation methods to analyze the impact of reservoir dam breaches on downstream urban areas by constructing dam breach models, hydrological models, and hydrodynamic models. The study reveals that dam breaches in small reservoirs in hilly areas can cause severe impacts on downstream cities, with the maximum flow velocity of breach floods exceeding 15 m/s, posing a significant threat to urban infrastructure and the safety of residents' lives and property. For reservoirs with larger catchment areas downstream, the combined effect of breach floods and rainfallinduced flash floods can result in dual impacts on the city. Such reservoirs, if breached, can lead to more severe disaster consequences and require prioritized prevention measures. This research can provide theoretical foundations and decisionmaking support for the safety management of small reservoirs in hilly areas and urban flood disaster mitigation.