This study investigates the failure modes of a shield tunnel passing through a layer of marine soft soil, employing the modified Cambridge model as the constitutive model for the soft soil. The objectives are to analyze the effects of varying groundwater levels and tunnel depths on the stress, strain, and to explore the characteristic failure modes during shield tunneling. A numerical computation model was established using the finite element software ABAQUS, simulating the processes of shield machine excavation, segment installation, and soil backfilling. The findings indicate that rising groundwater levels decrease the equivalent and shear stresses in the surrounding rocks, whereas increasing tunnel depth boosts both the equivalent and shear stresses in the surrounding rock. Settlement distribution around the tunnel is asymmetrical, with greater settlement at the top and lesser settlement at the bottom. The research provides insights for the design, construction, and safety assessment of shield tunnels within marine soft soil layers.