In order to study the change rule of reliability index of arch dam under seismic load,after the analysis of mode state and seismic dynamic for of a typical arch dam example based on ANSYS,the paper used the quadratic response surface method to calculate reliability index of each point of arch dam of the 90th calculation step under the action of earthquake load.The result is that it should be paid more attention that dam shoulders being under the action of earthquake load is taken as a weak tensile part . The change rule of reliability index of the middle part of arch dam surface upstream is complex and not to simply reduce.The result can provide a theoretical support for dynamic design and reinforcement measure of arch dam.