<p>Soil nonlinearity and the oblique incidence of seismic waves play crucial roles in the terrain amplification effect observed in slopes. To systematically investigate the underlying mechanisms and general behaviors of terrain amplification in soil slopes subjected to the coupled effects of nonlinearity and oblique incidence, a two-dimensional numerical model was developed. This model incorporates viscoelastic artificial boundaries and an equivalent linear viscoelastic constitutive approach. Through a parametric study, the effects of soil nonlinearity and oblique incidence on terrain amplification were evaluated across a range of slope angles, shear wave velocities, and input motion amplitude. The results indicate that soil nonlinearity generally diminishes terrain-induced amplification and shifts the fundamental site frequency to lower values. Oblique incidence, relative to vertical incidence, not only intensifies amplification effects but also alters the resonance frequency. Compared with acceleration amplification factor, spectral amplification factor is generally more conservative and better capture frequency-dependent amplification across the response spectrum. Neglecting soil nonlinearity in the Chinese seismic code GB50011-2010 leads to a 6.56% overestimation of the horizontal acceleration amplification factor. These findings offer valuable contributions to increasing the accuracy of seismic geohazard risk assessments and informing advancements in seismic design codes.</p>