Abstract: Beijing Municipality has numerous rivers along which many key high-risk enterprises are situated. Coupled with hazardous chemical transport across and alongside these waterways, this makes the region highly susceptible to sudden water pollution incidents. To accurately predict pollutant migration and dispersion for emergency response to sudden incidents, the upper Baihe River Basin (Beijing section) upstream of Miyun Reservoir was selected as the study area. Through environmental risk information collection during 2022-2025 and field surveys, key parameters including risk sources, risk receptors, hydrological and water quality data, and submerged topography were systematically obtained. A one-dimensional and two-dimensional hydrodynamic-water quality coupled simulation model for sudden water pollution incidents in the Baihe River Basin was constructed using the MIKE Model. A typical scenario involving toluene leakage from a transport vehicle on an upstream highway was simulated, enabling precise prediction of the arrival time of the pollution plume at key downstream cross-sections and Miyun Reservoir, along with its spatiotemporal concentration variations and peak concentrations. The leakage scenario in this study was defined as follows: under average-water-period hydrological conditions, two 30 t toluene tankers ruptured and leaked due to a landslide on a highway located 13 km upstream of the Beijing municipal border along the Baihe River, with a leakage flow rate of 0.1245 m³/s lasting for 553 seconds. Simulation results indicated that without emergency measures, leaked pollutants would reach Miyun Reservoir within approximately 76 hours. The peak toluene concentration at the Baihe Bridge inlet section would reach 94.89 mg/L (exceeding the 0.7 mg/L limit for toluene in surface water sources for centralized domestic drinking water under Environmental Quality Standards for Surface Water (GB 3838-2002) by approximately 136 times), posing a severe threat to Beijing's drinking water sources. Implementing integrated measures combining interception-adsorption with hydraulic regulation could delay the arrival of the pollution plume at Miyun Reservoir to 127 hours, significantly reducing the peak concentration at the Baihe Bridge section to 4.29 mg/L (exceeding the standard limit by approximately 6 times), thereby substantially mitigating environmental risks. Accordingly, it was proposed that emergency response should adhere to the principle of prioritizing source reduction supplemented by hydraulic regulation. By strengthening measures such as interception and adsorption to reduce the total amount of pollutants entering rivers, and complementing this with scientific hydraulic control, the safety of the capital's water environment could be most effectively safeguarded. The study demonstrates that the MIKE model can provide effective technical support for emergency decision-making in response to sudden river water pollution incidents.