In this paper, the penetrating crack in gravity dam's upper area is supposed to be a horizontal straight one, and the Newmark sliding-block method combined with the elastic time-history analysis of finite element method(FEM) is used to approximately compute the upper dam block's slipping displacement in earthquake, then based on the compata- tion results, the dam's seismic capacity is evaluated. In order to check the method in this paper, the results are compared to the ones computed by the FEM contact method. According to the results, it is shown that the dam in such a badly damaged condition would contain some seismic capacity, and the method used in this paper could generate good results under proper conditions, so the method could be used to approximately compute the slipping displacement of the destroyed dam's upper block in earthquake.%假设坝头处的贯穿性裂缝为一水平直线型裂缝，尝试用Newmark滑块位移法和普通的线弹性有限元时程分析相结合，近似计算这种带有贯穿性裂缝重力坝的上部孤立坝体在地震荷载中产生的滑动位移，评估了其抗震性能，并将Newmark法所得的结果与有限元接触模型的结果做了对比。计算结果表明这种受到损害的坝体确实具备一定的抗震能力，Newmark滑块位移法在合适的条件下能得到很好的结果，可用于近似计算已受损的重力坝上部孤立坝体在地震荷载中产生的滑动位移。
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