For the heightening designing of tailings dam, the dam's stability assessment is needed. In accordance with the borehole data, by combined with the heightening design scheme, the geological model of the tailings dam is established. Tailings dam's seepage stability, static stability, and dynamic stability under the flood and earthquake work condition are calculated. The results show that there is no infiltration of the overflow from the slope and the seepage is stable; the safety factor meets the regulatory requirements and the dam is in the static stability; dynamic responses and liquefaction results of the dam show that the dam is stable. A comprehensive assessment of the stability line is provided. It possesses certain significance in the evaluation of seepage, static stability, and dynamic stability based on the principle of effective stress.