To investigate the long-term stability under marine ion erosion of the semi-curtain grouting reinforcement zone in submarine tunnels, an accelerated laboratory testing method was first developed based on Fick′s second law of diffusion and the Life-365 standard design protocol. By adjusting the surface ion concentration and curing temperature, the long-term deterioration of grouted material under seawater ion intrusion was effectively reproduced within the duration of a short experiment. Subsequently, a series of uniaxial compression tests under ion erosion conditions were conducted to quantify the deterioration characteristics of grouted specimens having varying water-cement ratios. Based on the concept of equivalent thickness reduction and damage evolution, a performance degradation model and a durability prediction model were established. A thickness reduction equation and a damage evolution equation were proposed, and a damage-based failure criterion was developed to assess the long-term risk of deterioration in the reinforcement zone. Finally, a multiphysics coupling model that considers material deterioration was established using COMSOL Multiphysics software. The proposed thickness reduction and damage evolution models were embedded to simulate the evolution of stress, deformation, and pore pressure in the semi-curtain grouting reinforcement zone of a typical water-rich sandy section of the Qingdao metro line 13 over an 80-year service period. The results indicate that seawater ions penetrate the structure omnidirectionally, rather than strictly from the exterior inward. At approximately 60 years of service, significant deformation and stress redistribution occur in both the grouting zone and the tunnel lining. The lining crown, invert, and the shoulder region of the grouting zone are identified as areas critical to the long-term durability of submarine tunnels subject to marine erosion.