The coupled 3D Dynamic Mechanical/Fluid is performed for the Nuozhadu earth and rock-filled dam by FLAC3D, but literature on the fully coupled of fluid-solid under earthquake is not too much. This paper gives a good example of applying FLAC3D to do the fully coupled simulation, and after a system in mechanical and fluid is got, the dynamic simulation can be done. A more accurate estimation of pore water pressure and the distribution of acceleration and irrecoverable displacement of the dam under dynamic are obtained. The result shows that permanent displacement would occur in the potential slide mass of the slope under earthquake. Finally the method to improve the slope stability is suggested. The results provide important references to the design.