The relative position of lateral karst cavities influences the seismic response of tunnel structures. This study numerically investigated the impact of cavities located behind a shield tunnel along the path of obliquely incident SV waves on dynamic response and structural damage. Using finite element analysis with viscous-spring artificial boundaries (validated for accurate seismic wave input at arbitrary angles), the effects of cavity diameter (D) and cavity-tunnel spacing on tunnel deformation and stress distribution were analyzed. Tunnel damage, assessed by correlating tensile damage factors and concrete crack widths, reveals the following. (1) Seismic response and damage are highly sensitive to incidence angle. At 0° incidence, tunnel deformation and stress distribution are considerably affected by cavity diameter and spacing; stress amplitude at the cavity-side arch shoulder varies by 40.99% and 77.78%, respectively, with cavity diameter mainly influencing deformation range. (2) At 30° incidence, the stress distribution is less sensitive to cavity diameter and spacing, but structural damage increases. With cavity diameter ≥0.8D and spacing ≤0.6D, damage concentrates at the invert center, shifting toward the arch foot. While lateral karst cavities can contribute to energy dissipation and vibration reduction, they also exacerbate localized damage. These findings provide valuable insights for the design and monitoring of tunnels in karst regions.