Using dynamic finite element numerical simulation, in this study, we establish models with different slope gradients (α=30°, 45°, 60°, and 75°). The incident direction of a seismic wave is along the direction of the tunnel axis. As such, we studied the influence of different slope gradients on the dynamic response of the tunnel portal section and the front slope. By setting a contrast model, a pure slope without any tunnel, we examined the influence of a tunnel on the dynamic slope response. The results show that: (1) In the presence of a free face, the displacement and acceleration of the portal section undergo an obvious amplification effect. The displacement peak values of different slope models are located in the section y=0 m (distance from the portal), and the displacement of each control point decreases rapidly 40 m from the entrance of the tunnel. (2) Along with the increase in slope gradient, the displacement value of the tunnel in the same section increases, and the deformation of the portal section also increases. (3) When α≥60°, with an increase in slope elevation, displacement increases; when α<60°, with an increase in slope elevation, the displacement first increases and then decreases. The peak displacement of the slope appears at 0.4~0.6 of the slope height, which means that with an increase in slope gradient, peak displacement is closer to the top of the slope. (4) The existence of a tunnel has a significant influence on slope stability, which is particularly evident in the vicinity of the portal.