Abstract: The occurrence of cascading dam-break floods can lead to devastating disasters in downstream areas. Current research on cascading dam-break floods of concrete dams has not focused on scenarios involving partial breaches of the dam. This study employed a combination of large-scale flume experiments and theoretical analysis to investigate the propagation mechanisms of dam-break floods passing through vertically partially breached dam structures. The results indicate that after the vertical partial breach of a second dam under the impact of an upstream dam-break flood, either a rising negative wave or a falling negative wave forms in its reservoir, thereby influencing the upstream flow depth hydrograph. The pressure load exerted by the undular wave on the downstream dam exhibits periodic oscillations, with the peak load from the bore waves being greater but rapidly decreasing thereafter. A simplified calculation method for the load exerted by dam-break floods on downstream dams was established, and the results agreed well with the experimental values. When the vertical breach height of the second dam is less than 0.2 times the dam height, it has a blocking effect; when it exceeds 0.2 times the dam height, it has an incentivizing effect. The findings of this study hold significant importance for the assessment of cascading dam-break flood disasters.