Based on“The Seismic Risk Analysis of Jiayuguan City”,this paper anticipates the seismic disas-ter of Dacaotan earth dam. We calculate and analyse the dam's nonlinear dynamic characteristics using dynamic nonlinear finite element method,and give the possible pull-open regions on the dam's surface and the possible plastic failure regions in the dam under l0% and 2% of exceedance probability during a period of 50 years,andgive the acceleration time history and shear stress time history of several rep-resentative points. We also calculate and analyse the whole stability of the dam. Under the two ex-ceedance probabilities,this paper gives the whole safe reliability during a period of 50 years.