Most dynamic response analyses of rock slopes only consider the vertical incidence of seismic waves. However, in near-field seismic areas, obliquely incident waves are predominant. In the 2008 Iwate-Miyagi earthquake, numerous large-scale landslides occurred, leading to significant loss of life and property. Among them, the Aratosawa landslide is considered one of the largest. This paper evaluates the comprehensive impact of the seismic wave incidence angle on landslides. Based on the Discontinuous Deformation Analysis (DDA) method and combined with viscous boundaries, an Aratosawa landslide model was established. By comparing the seismic waves of vertical and oblique incidence, the occurrence, destruction, and consequences of the Aratosawa landslide were simulated. The results show that obliquely incident seismic waves lead to a larger seismic response. Furthermore, the slope failure simulation, considering the wave incident angle, can be well-analyzed using the DDA method.