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The stable time for concrete permeability is of great significance to evaluate the permeability of concrete and predict the service life of actual engineering. Based on a test with an exposure time of more than 3 years in a natural tidal environment, the gas permeability and microstructure parameters of concrete with different water-binder ratios (w/b) and admixtures including fly ash (FA), slag (SG), silica fume (SF), and basalt fiber (BF) were tracked and tested. Then, the time-dependent gas permeability and main microstructure parameters and their stable time were investigated. Finally, the relationship between stable time for the gas permeability coefficient and microstructure parameters was studied. The results show that the apparent and intrinsic gas permeability coefficients of concrete both decrease with exposure time. For ordinary concrete, the gas permeability coefficient and the corresponding stable time increase with w/b. The stable time for gas permeability coefficients of SF concrete is the shortest among admixture concrete, while that of BF concrete is the longest. The timedependent gas permeability is in keeping with the change of total porosity and contributive porosity of large capillary pores (100 to 1000 nm) with exposure time. Moreover, the change trends of stable time for gas permeability, total porosity, and contributive porosity of large capillary pores with w/b and admixture type are consistent. The stable time for the intrinsic gas permeability coefficient has a better correlation with that for contributive porosity of large capillary pores than that for total porosity. The intrinsic gas permeability coefficient stabilizes first, followed by contributive porosity of large capillary pores, which is the most important factor influencing the stable time for the intrinsic gas permeability coefficient, and total porosity stabilizes last. [ABSTRACT FROM AUTHOR]