<p>There are many potential seismic sources around a slope in earthquake zones. Thus, the seismic waves could come to the slope from arbitrary directions. However, few researches have been conducted on the influences of seismic wave incident directions on slope seismic resistance. In this paper, the concept of critical peak acceleration (CPA) of seismic ground motion acting on a slope was proposed to characterize the slope seismic resistance, and the CPA was confirmed in light of penetration of the plastic zones in the slope body. The CPAs of Panlongyuan loess slope located in Baoji City against actions of seismic waves of simple harmonic P waves and S waves with different incident directions were searched by load increasing method with Flac^3D software. Accordingly, the influences of seismic wave incident directions were discussed. The results show that seismic wave incident direction has significant effects on the CPAs of the slope. The CPAs of the slope against the actions of seismic waves coming from the back of the slope are relatively smaller, so the slope is more prone to failure. Therefore, the seismic wave incident direction should be considered in the analysis of slope dynamic stability. In addition, the CPAs of slopes against P waves are also different from those against S waves. So, to evaluate the stability of a slope subjected to a real seismic wave, the seismic phases of incident waves should not be ignored. The research of this paper reveals the effects of seismic wave incident directions on the slope CPAs and paves a way for the probability analysis of seismic-induced landslides.</p>