<p>Continuous downward water migration occurs during tunnel construction when a soft-plastic loess layer is overlaid on a large cross-section tunnel. Accordingly, the unique water sensitivity of loess often results in construction challenges. In this paper, the characteristics and influencing factors of water migration in a large cross-section tunnel overlaid by a soft-plastic loess layer were studied. First, the seepage–stress theoretical interaction model of a soft-plastic loess tunnel was established. Triaxial unloading–seepage tests were then conducted on soft-plastic loess using a triaxial test platform developed by the authors. Test equations were obtained for embedding into a 3D fluid-mechanical coupled numerical analysis. The water migration characteristics were studied by performing a series of numerical simulations, and TDR moisture monitoring was performed for validation. The results revealed two water migration paths in tunnels overlaid by a soft-plastic loess layer. The first is from the arch crown to the inverted arch along the back of the primary support, and the second is along the surrounding loess of the core soils. When the thick of overlain soft-plastic loess layer is greater than 4&#xa0;m, the blocking effect of the primary support cuts off the migration path of the core soil. In contrast, when the thick of overlain soft-plastic loess layer is greater than 10&#xa0;m, water in the soft-plastic loess layer will not migrate to the tunnel during construction.</p>