Abstract: There are many dangerous reservoirs in China, and the dam failure risk is very high under extreme weather and other emergency events in local regions. Taking a reservoir in Shandong Province as the research object, this paper calculates the break discharge, simulates the flood routing, analyzes the overtopping break process and impact, and draws the inundation map using gradual break model and two-dimensional hydrodynamic model. The results can provide decision support for the reservoir management organization to deal with the flood risk of dam breach.