The evolution of dam-break floods in downstream and cascade reservoirs is an extremely complicated process. Based on the three-dimensional Navier–Stokes equation and smoothed particle hydrodynamics theory, the complicated dam-break flood flow was simulated for such cases. The evolution of the dam-break flood in the downstream reservoir and cascade reservoirs and its impact on the downstream dam were obtained, and the results were found to agree well with experimental results. According to force analysis, the dam-break flood effect can be divided into a high-speed impact area and a pressure-transmission area. The former can cause damage due to instantaneous impact on the dam and the latter can cause damage arising from oscillation fatigue. These conclusions have value in water resources and hydropower engineering for preventing the outburst of a cascade reservoir group.