When　the　numerical　analysis　of　a　long　lined　tunnel　is　carried　out,　the　calculation　amount　of　the　finite　element　model　becomes　restricted　large-scale　parameter　analysis.　In　this　paper,　an　efficient　and　high-precision　2.5-dimensional　(2.5D)　frequency-domain　finite　element　method　is　used　to　simulate　the　three-dimensional　response　of　tunnels　under　the　action　of　oblique　incident　plane　seismic　waves.　This　method　can　save　calculations　and　avoid　the　boundary　effect　caused　by　the　longitudinal　truncation　of　the　tunnel.　The　2.5D　zigzag-paraxial　boundary　is　developed.　The　artificial　boundary　is　attached　to　the　structure’s　surface.　The　substructure　method　for　oblique　plane　seismic　waves　is　established.　Comparing　the　substructure　method　with　the　analytical　solution,　the　correctness　of　the　site　response　is　verified　first.　The　accuracy　of　the　2.5D　finite　element　substructure　method　is　further　verified.　The　parameter　analysis　of　different　incident　angles　and　conversion　angles　shows　that　the　underground　tunnel　does　not　reach　the　maximum　of　structural　seismic　response　when　the　seismic　wave　is　vertically　incident.　The　location　of　the　soil–rock　interface　on　the　tunnel　is　further　discussed.　The　results　show　that　when　the　underground　tunnel　crosses　the　location　of　the　soil–rock　interface,　the　seismic　response　of　the　tunnel　will　be　amplified.　©　2023　by　the　authors.