The chloride ion migration and corrosion mechanisms are complicated in real marine corrosive environments. This paper focused on a low carbon cementitious composite system of alkali-activated slag-limestone powder (AAS-LS). The chloride transport properties of AAS-LS concrete were evaluated by RCM and real marine exposure tests. The effects of various areas of the marine environment and LS content on the microstructure evolution mechanism of AAS-LS system were analyzed by XRD, TG-DTG, FTIR and SEM-EDS. The main conclusions include the maximum chloride concentration in the splash zone was approximately 4–5 times that in the atmosphere zone after 3 months of exposure. Compared with the results of RCM at 90 d, the chloride ion diffusion coefficients obtained from the two methods were in accordance with S > L5S > L15S > L10S. LS (within 15 wt%) has the effect of hydration, nucleation, filling and dilution in AAS concrete, and has no bad effect on chloride transport properties. More severe carbonation was observed in the marine atmospheric zone with a polycrystalline phase of vaterite and aragonite, while Mg(OH)₂ was formed under seawater immersion. The further hydration of slag was promoted by seawater immersion.