Abstract: If the dam failure and break induced by overtopping flow and leakage through the dam, it will cause a great loss of life and property for the people downstream. Therefore, it is significant to predicate accurately the flooding area. By analysis of 22 hydropower stations located on the Dadu River, Changheba hydropower station was selected as a research object. The flooding area of Luding county caused by the overtopping floods of the Changheba hydropower station dam and its downstream Huangjinping dam and Luding dam was analyzed in this paper. Firstly, the terrain data was extracted by HEC-GeoRAS and Google Earth, after then, the data was imported into the HEC-RAS and dam-break flood was calculated. Finally, the flooding area map and flow velocity distribution map were captured using HEC-GeoRAS. The analysis results show that there is a flooding area of 200 m induced by overtopping along both left and right bank sides; the flow velocity along the center of the river is large and the maximum velocity can reach 16.217 m/s. The analysis results can provide a technical support for the production of a flood risk chart and for flood control decision.