Abstract: Coastal high-speed railway tunnel linings that are persistently exposed to high-concentration salt spray and hot–humid marine environments are subject to an elevated risk of accelerated deterioration. Aerodynamic disturbances induced by train operations further promote the migration and spatial redistribution of salt spray, directly affecting protection zoning and design parameters. To address this issue, a coupled numerical model of the train–tunnel–salt spray system is established. Numerical simulations are conducted to investigate the dispersion characteristics of salt spray under high-concentration conditions at both the tunnel portal and within the tunnel, considering single-train passage and train-meeting scenarios. The results indicate that salt spray exhibits pronounced spatiotemporal evolution characteristics at the tunnel portal and inside the tunnel. At the portal, salt spray dispersion shows a characteristic three-stage evolution, and train-meeting conditions significantly intensify the dynamic fluctuations in salt spray concentration. Inside the tunnel, salt spray dispersion is dominated by wake entrainment and shear effects, resulting in a distinctly three-dimensional distribution with pronounced lateral heterogeneity.