To more accurately simulate the inundation process of inundation areas downstream of small-sized reservoirs after dam breaches and reveal the coupling mechanisms of dam breaches and levee overtopping, the DaYJ Reservoir in Guizhou Province was studied. Digital elevation measurements and orthophoto imaging were conducted in the reservoir area and downstream flood-prone areas. A coupled model of dam breaches and levee overtopping was developed based on a high-precision DEM, and sensitivity analyses were performed within the study area regarding flood depth, flow velocity, and flood severity relative to levee height. The results indicate that compared with the single dam breach model, the coupled model of dam breach and levee overtopping can more realistically simulate the changes in the inundation range downstream over time. When the levee height is no more than 1 m, overtopping is highly probable. A transitional state between levee overtopping and non-overtopping occurs at a levee height of approximately 1.5 m. When the levee height exceeds 1.7 m, overtopping is entirely prevented, and the flood primarily bypasses the levee from its end, inundating residential areas. As levee height increases, the arrival time of the flood, the emergence time of a 0.3 m water depth, and the occurrence time of the peak flow velocity are all delayed. Furthermore, both the maximum flow velocity and the peak flood severity within the study area of levee overtopping generally decrease and become more uniformly distributed.