In order to investigate the seismic response characteristics and seismic fortification range of fault-crossing tunnels in strong earthquake zones, a pipeline tunnel crossing a fault in western China was used as a case study. The energy-based analysis method was introduced into the study of the dynamic response of fault-crossing tunnels, and an energy balance equation under seismic loading was established. The finite difference numerical simulation method was used to explore the longitudinal stress response, strain energy distribution characteristics, and seismic fortification range of the tunnel structure under earthquake action. The results show that stress mutations occur at various parts of the tunnel structure when crossing the fault zone, with the most pronounced stress concentration observed at the vault. The elastic strain energy density reaches its peak near the fault interface, showing significant energy distribution heterogeneity. The plastic strain energy gradually accumulates within the tunnel structure, intuitively revealing the damage evolution process at critical locations such as the vault and the invert. By comprehensively analyzing the minimum principal stress amplification factor, maximum shear stress, and elastic strain energy density, the seismic fortification range for the fault-crossing tunnel is proposed: 2.4D~2.6D for the hanging wall and 2.5D~2.6D for the footwall. It is concluded that the stress and energy responses of the tunnel under seismic loading are significantly affected by the fault structure. The application of energy-based analysis provides a rational definition of the seismic fortification range and offers valuable insights for the seismic design of fault-crossing tunnels in strong earthquake regions.