During seismicity, seismic acceleration first increases then decreases. Using a pseudo-static method and considering the acceleration change during earthquakes, sliding and toppling failure modes were calculated for the unstable rock on Tianlong mountain, Taiyuan city, Shanxi province. We found that the failure mode transformed from sliding to toppling under seismic action. Extending this problem to a general situation, we conclude that: the contribution of seismic forces to unstable rock damage differs and the overturning moment is relatively larger than the sliding force; for stability evaluation, the seismic action process must be considered, and the failure mode of unstable rock whose safety factor first reaches 1.0 should be used to calculate the possible mode; to ensure the stability of unstable rock of different risk states under seismic action, the seismic strengthening design should consider different possible failure modes.