Abstract: Reservoir dam break flood is characterized by its sudden onset, intense destructive power, and wide impact range.Scientifically assessing its risk is therefore crucial for effective disaster prevention and mitigation in downstream regions.However, while existing 1D models cannot capture lateral flood dynamics, conventional 2D models lack the computational efficiency required for real-time early warning applications.Therefore, taking the Dagaotan Reservoir as an example, a GPU-accelerated 2D model is combined with the progressive dam-break boundary conditions to construct an efficient and precise method for simulating downstream flood and loss assessment of the reservoir.This method takes into account physical properties of dam materials and uses the gradually collapsing dam model to calculate the breach flow under different dam-break conditions as the flow boundary of the 2D hydrodynamic model.Then, high-resolution DEM data are used to define the grid of the hydrodynamic model, land use data is used to define the roughness coefficient of the model, and the flood risk factors such as inundation water depth, inundation extent and inundation duration are simulated.The spatial weight overlay analysis method based on GIS is used to evaluate the economic losses.The results show that the full-dam-break flood in the downstream of Dagaotan Reservoir first spreads to the low-lying areas, where water accumulates, then the inundation range and water depth gradually expand until reaching a maximum, after which they gradually recede.The evolution process of regional floods follows the pattern of strong erosion in the upstream and flood buffering in the downstream.The peak water depth at the characteristic points in the upstream occurs earlier, is higher, and the accumulated water can fully subside; the peak water depth at the characteristic points in the downstream occurs later, is lower, and the accumulated water can also subside.The low-lying areas along the river continue to be stagnant for 24 hours due to terrain influence.Under the three scenarios of local dam-break during design flood level, local dam-break during check flood level, and full-dam-break, the inundation water depth and area increase progressively, and the significant increase in the total inundation area mainly results from the expansion of area with high water depth.The severity of dam-break is positively correlated with economic losses.The economic losses caused by the full-dam-break scheme (439 million yuan) are far greater than those of the local dam-break scheme, and its destructive impact on residential areas and agriculture is particularly prominent.The research results can provide a reference for flood prevention and emergency evacuation in the downstream of Dagaotan Reservoir, and can also provide support for relevant departments to formulate scientific emergency plans and disaster reduction measures.