<p>The evolution mechanism of narrow crack in tunnel lining concrete under seepage and dissolution effects remains unclear. To address this issue, seepage–dissolution tests are conducted using a custom-designed device, with static dissolution tests used as controls. Using 3D laser scanning and scanning electron microscopy (SEM), the changes in concrete crack surfaces are analyzed from both qualitative and quantitative aspects, ultimately revealing the evolution mechanism of narrow concrete cracks under seepage and dissolution effects. Research results indicate that both static dissolution and seepage–dissolution have a significant influence on concrete crack surfaces, including the appearance of calcium carbonate (CaCO₃), the peeling of fine aggregates and the exposure of coarse aggregates. An increase in seepage velocity inhibits the formation of CaCO₃ on the crack surface. After 30&#xa0;days of static dissolution and seepage–dissolution at a seepage velocity of 5 cm/s, the dissolution thicknesses of the crack surface are 0.048 mm and 0.0893 mm, respectively. The dissolution thickness increases linearly with the rising seepage velocity, with a correlation coefficient greater than 0.97. Influenced by the composition of the concrete around the crack, the fractal dimension of the crack surface may increase or decrease during dissolution. The dissolution of substance like calcium hydroxide (Ca(OH)₂) results in issues such as structural loosening and fine aggregate peeling, which further induce crack width expansion. Meanwhile, the isolated C–S–H gel can re-bond substances, such as peeled fine particles, CaCO₃ crystals and other precipitates, and thereby blocking the narrow crack. These two competing width evolution mechanisms, dissolution-driven expansion and cementation-driven blockage, are closely related to seepage velocity and result in three possible evolutionary trends for narrow concrete cracks, i.e., expansion, no change or blockage.</p>