<p>To effectively assess the impact of time-dependent characteristics of unsteady seepage on the stability of the tunnel excavation face, this study derives an analytical expression for the ultimate support pressure of the tunnel excavation face under unsteady seepage conditions. It investigates the variations in excavation face support pressure and soil failure modes with respect to time. The results show that under unsteady seepage conditions, the ultimate support pressure of the tunnel excavation face decreases over time and eventually stabilizes, with its value falling between that of the static water condition and steady-state seepage. As the seepage time increases, the shape of the instability failure mode continues to evolve along the tunnel excavation direction. Furthermore, the increase in soil strength parameters (soil cohesion and internal friction angle) significantly reduces the ultimate support pressure and the range of soil instability failure, with a more pronounced effect than the conductivity coefficient. In practical engineering, for high permeability soil layers (such as sandy soil and gravelly soil), optimizing the support pressure design by considering the soil conductivity coefficient can provide a more accurate reference for support pressure of the tunnel excavation face in underwater tunnel projects, thus enhancing the economic efficiency of the project while ensuring safety.</p>