Abstract: During construction and operation, immersed tunnels in inland waterways are subjected to coupled environmental factors, including ionic erosion from river water (chemical field), overburden soil and hydrostatic pressure (stress field), and flow-induced scouring (seepage field), all of which contribute to the deterioration of concrete mechanical properties and threaten structural integrity and service life. To address this issue, this study investigates the concrete degradation mechanisms in the Shunde Lungui Road submerged tunnel under multi-field coupling conditions. Through long-term stress-seepage-chemical erosion experiments at varying flow velocities, the mechanical performance degradation patterns of tunnel concrete were systematically analyzed using micro-indentation techniques. Additionally, scanning electron microscopy (SEM) and mineralogical analysis were employed to elucidate the influence of flow velocity on concrete deterioration. Key findings include: (1) Under inland water erosion, the elastic modulus of tunnel concrete initially increases before weakening, primarily due to variations in the content of reaction products such as gypsum, ettringite, and Friedel’s salt; (2) Increasing flow velocity accelerates erosion progression, with erosion depth rising from 4.42 mm at 0 m/s to 5.66 mm at 0.06 m/s, and the erosion rate increasing from 0.0587 mm/day to 0.0783 mm/day; (3) Higher flow velocities exacerbate concrete degradation, reducing the near-surface elastic modulus from 17.86 GPa at 0 m/s to 16.41 GPa at 0.6 m/s. This study reveals the critical role of flow velocity in the mechanical deterioration of immersed tunnel concrete in complex inland environments, providing valuable insights for the design, construction, and maintenance of such infrastructure.