<p>Seismic fault zones are widely distributed worldwide and tectonic compression often gives rise to geologically complex mountain ranges. In such mountainous areas, where rugged terrain and weak geological conditions prevail, twin-arch tunnels are often the only viable solution for dual-line highways or railways due to limited construction space and alignment constraints. When these tunnels cross near-fault seismic zones, they encounter substantial earthquake-engineering difficulties, especially from the long-period pulse effects and intense vertical ground motions characteristic of near-fault earthquakes. However, existing studies lack a theoretical method specifically suited to the assessment of seismic performance concerning near-fault twin-arch tunnels. To address this gap, this study first developed a seismic mechanical model specifically designed for twin-arch tunnels. Next, numerical simulation was combined to obtain methods for determining the relevant computational parameters of the theoretical model. Subsequently, the validity of the seismic calculation method for twin-arch tunnels was confirmed through numerical simulations and physical model tests. Lastly, a parametric analysis was performed using the proposed methodology. The analysis reveals a nuanced relationship between the structural geometry and stress response of the twin-arch tunnel. Moderately increasing the centre wall’s width-to-height ratio is shown to alleviate stress concentrations therein. Conversely, an excessive ratio exacerbates both tensile and compressive stresses within the primary tunnel liner. Regarding geotechnical and seismic parameters, a greater burial depth improves structural stability by inducing higher compressive stress, thereby suppressing tensile failure. Finally, the horizontal PGV/PGA ratio exerts a limited influence on the inner liner but has a more pronounced impact on the outer liner and the centre wall.</p>