Abstract: Near-fault pulse-like ground motions with long-period velocity pulses may cause severe damage to subsea shield tunnel structures. However, the longitudinal seismic response analysis of such tunnels poses significant computational challenges due to large spatial scales, densely assembled segmental linings with numerous bolts, and strongly nonlinear soil-structure interaction, rendering refined longitudinal models low computationally feasibility. To address this, a dynamic substructure approach based on the response displacement method is developed for the 2.7 km-long Su'ai submarine shield tunnel. Near-fault pulse-like and far-field ground motions are employed to investigate the differences in longitudinal seismic response, and an optimal intensity measure is identified to characterize the tunnel's longitudinal damage potential. Results indicate that near-fault ground motions, characterized by significant velocity pulse effects, induce substantially more severe seismic responses compared to far-field excitations. The structural longitudinal response demonstrates increased sensitivity to low-frequency seismic waves, with distinct spatial distributions of seismic stress: peak stresses concentrate at the tunnel haunch under near-fault motions, whereas they shift to the abutment under far-field conditions. Root-mean-square velocity (RMSV) is recommended as the optimal parameter for characterizing the structural damage potential of subsea shield tunnels.