Abstract: The randomness of seismic motion significantly affects the dynamic response of dams. The ultimate capacity is a critical factor for the seismic safety of dams, thus during the assessment process the influences of load randomness should be thoroughly considered. Based on a large number of random dynamic finite element calculations for high earth-rock dams, the relationship between the settlement rate of dam crest and the cumulative sliding displacement of slope, two important deformation indices of dams is studied. Additionally, the safety control standards for earth-rock dams are proposed. The probabilistic density evolution method is used to analyze the ultimate a seismic capacity of dams. The results indicate that the settlement rate increases with the cumulative sliding displacement and follows a logistic growth model. Taking the exceedance probability of 10% as an example, the ultimate a seismic capacities for a 250 m-high faced rockfill dam and a high-core rockfill dam are approximately 0.7g~0.75g and 0.8g~0.85g, respectively. A new probabilistic approach is proposed for evaluating the ultimate a seismic capacities of dams.