The microstructure evolution and stress-coupling failure of copper alloys are the primary factors contributing to their accelerated degradation in deep-sea environments, particularly under seawater pressure. In this study, high-manganese aluminum bronze (MAB), a material commonly used in ship propellers, is selected as the research subject. The atomic structure model of MAB alloy with aluminum atomic surface segregation and the strain-coupling model under varying seawater pressures are established through a combination of simulation and experimentation. The failure mechanism under different pressures is further examined, and the corrosion rate is predicted. As seawater pressure increases from 0.1 MPa to 10.0 MPa, the alloy’s dislocation density and strain level progressively rise. When the pressure exceeds 6.0 MPa, these increases become more pronounced. The dislocation density of the alloy’s (111) crystal surface increases by 6.08 × 10⁻³ nm⁻², while the elastic micro-strain level rises by 0.79%. Experimental results indicate that the weight loss rate of copper alloys escalates with increasing pressure. Additionally, the morphology of the corrosion products transitions from a spot-like to a lamellar structure. Density functional theory calculations reveal that seawater pressure significantly affects the stability of the alloy structure. When seawater pressure surpasses the critical threshold of 6.0 MPa, the copper vacancy formation energy and migration–dissolution activation barrier decrease substantially, leading to a notable increase in the dissolution rate. The molecular dynamics method is employed to investigate the adsorption behavior of Cl⁻ on the alloy surface under deep-sea pressure and its influence on corrosion. The results demonstrate that increasing pressure enhances the chemical adsorption of Cl⁻. However, at 10.0 MPa, the saturated adsorption of Cl⁻ on the surface does not significantly affect the dissolution rate.