[Objective] The simulation method of flood evolution process caused by reservoir dam failures in urban areas was analyzed, in order to provide a scientific basis for risk prevention, early warning system development, and evacuation plans. [Methods] Using the upstream Jiangang and Changzhuang reservoirs in Zhengzhou City as case studies, a high-resolution digital elevation model was constructed based on GIS technology. The MIKE FLOOD coupling model was employed to integrate one-dimensional and two-dimensional models of reservoir dam failure, simulating the flood evolution process and exploring the characteristics of floods in downstream rivers and complex urban areas. [Results] Immediately following the dam failure, the peak discharge at the breach quickly reached its maximum. Peak outflow volumes from the Changzhuang and Jiangan reservoirs were 4 542.03 and 17 549.1 m³/s, respectively, with non-converging flood peaks. Portions of both banks downstream of the Jialu River, within 15.00 to 31.39 km from the Jiangan reservoir, were inundated, covering an area of 56.37 km². One hour after the breach of the Changzhuang reservoir, riverbanks began to overflow, resulting in increased inundation areas. The Jiangan reservoir dam failure occurred 4.25 h later. 1.25 h after the Jiangan reservoir breach, inundation depths significantly increased in the original inundation area, expanding further to the maximum inundation area 1.92 h later. Following the dam breaks of the Changzhuang and Jiangan reservoirs, the floodwater level rose above the top of the south-to-north water transfer protection embankment, and a large volume of floodwater surged into the main channel of the south-to-north water transfer, generally increasing the flood flow rate. [Conclusion] The peak flood flow from the Jiangan reservoir is significantly larger than that from the Changzhuang reservoir, making its dam break more harmful. After the dam breaks, a substantial amount of floodwater enters the main channel of the south-to-north water diversion Project, increasing the risk of erosion of dikes, slopes, and inlet and outlet lock chambers. This situation seriously affects the water conveyance safety of the main canal of the south-to-north water diversion project and the safety of people’s lives and property.