The spillway of Yingdeer Reservoir, located on the dam, was broken down in the morning on April 28,2005, and it was called the right auxiliary - dam collapsing event. Before the dam break, two earthquakes occurred. To analyze the effect of the earthquake on the dam break, the theory of load/unload response ratio （LURR） is used to study the dynamic response law of the dam body. The result shows that the dam was stable in the twice earthquakes. The seismic stability analysis shows that the earthquakes are the main causes to quicken the spillway to break down.