Abstract: In order to study the occurrence and development process of the "2022.8" levee-breach flood in the downstream of the Raoyang River, the causes of the flood and the levee- breach were analyzed. The "2022.8" flood process in the downstream of the Raoyang River was simulated by using a two- dimensional unsteady flow hydrodynamic model. The changes of river regime from the Raoyang River to the Liaohe River estuary, the flood relationship between the Raoyang River and the Liaohe River and the causes of the Shusi section levee-breach of the left embankment of the Raoyang River are analyzed. The results show that the inundation area calculated by the models is basically consistent with the results of remote sensing monitoring. The Liaohe River flood in 2022 has the blockage effect on the lower reaches of the Raoyang River. The blockage water level at the estuary of the Raoyang River increases and the flow velocity decreases obviously. The blocking effect of the Liaohe River flood from the estuary of the Raoyang River to the upstream decreases gradually, and the impact has become small at the Yuhegai Station. The analysis of river regime changes at the estuary of the Raoyang River and the comparative analysis of flood processes at relevant stations in recent three years also confirm this result. It can be seen from the cause analysis that it is inevitable to break the levee at the Shusi section of the left embankment of the Raoyang River. Suggestions on measures are put forward for the results of simulation and analysis.