[Objective] This study aims to scientifically assess the risk of flood diversion operation in the Dongxihu flood detention basin, the only remaining flood detention basin around Wuhan. The study systematically analyzes the inundation characteristics, flood-control benefits, and socioeconomic risks associated with flood diversion, ultimately enhancing flood prevention and disaster mitigation capacity in the Yangtze River Basin. [Methods] This study employs MIKE series models to develop a coupled hydrodynamic model combining 1-D simulation for the main channel of the Yangtze River and 2-D local simulation for the Dongxihu flood detention basin, enabling refined simulation of flood propagation and inundation processes. The 1-D hydrodynamic model covers the Yangtze River reach from Yichang to Datong, encompassing the entire middle and lower Yangtze River basin including Dongting Lake and Poyang Lake. The computational domain of the 2-D model includes the Hanjiang River reach from Hanchuan station to Jianghan bridge, as well as the Dongxihu flood detention basin. The 1954-type 300-year return period flood, an over-standard flood in the middle and lower reaches of the Yangtze River, is selected as the hydraulic boundary condition. Simulations were performed to analyze the inundation extent, water depth distribution, flood storage volume, water level variation at Hankou, together with the exposure and losses of population, properties, enterprises and other socio-economic assets after flood diversion in the Dongxihu flood detention basin. [Results] After flood diversion operation in the Dongxihu flood detention basin, the final flood storage volume in the area reaches 2.374 billion m³. The maximum inundation area reaches 391.77 km², with a maximum inundation depth of 7.46 m and an average inundation depth of 3.82 m. During the entire flood diversion process, a total of 2.374 billion m³ of floodwater from the main stream of the Yangtze River is diverted, effectively lowering the water level at Hankou by 0.33 m, while the maximum water level reduction at Hankou during the whole flood process reaches 0.41 m. The inundated area is mainly concentrated in subdistricts in the central-western and northern parts. Although the eastern and central subdistricts have relatively smaller inundated areas, their population and property exposure is extremely high, indicating that flood risk is not simply proportional to inundation extent but is strongly influenced by the spatial distribution of socioeconomic assets. Areas such as the Duoluokou port area of Wuhan port, Wuhuan sports center, and Baiquan subdistrict are not affected by inundation after flood diversion because of their higher terrain. Since numerous buildings are distributed near the flood diversion entrance, relatively high flow velocities occur near some buildings after diversion, exerting considerable impact forces on these structures. Flood diversion would inundate 169.05 km² of cultivated land, necessitate the evacuation of 955 000 people for risk avoidance, cause direct economic losses of 105.254 billion, and lead to indirect economic losses also reach the hundreds of billion scale. These indirect losses, resulting from large-scale enterprise shutdowns and supply chain disruptions, may exceed direct losses and therefore deserve particular attention in risk assessment, compensation policy design, and resilience planning. [Conclusions] The Dongxihu flood detention basin performs a critical flood impoundment function, yet flood diversion entails heavy costs and marked spatial variability in risks. Future planning should coordinate flood diversion requirements with territorial spatial development layout, optimize the allocation of population and industries, and minimize losses induced by flood diversion operation. Future studies should incorporate more detailed and time-varying socioeconomic data, improve model generalization and uncertainty analysis, and strengthen the coupling of hydrodynamic simulation with dynamic risk assessment so as to support more adaptive and resilient flood risk governance.