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Based on the exposure test of fly ash (FA) concrete in different environments, chloride diffusion coefficients and main microstructural parameters measured by mercury intrusion porosimetry (MIP) and their time dependent properties were analyzed at first. Then, the effect of FA content on time-dependent chloride diffusivity and microstructural parameters was investigated. Finally, according to the similarity theory and Jaccard coefficient, the similarities of instantaneous chloride diffusion coefficient and main microstructural parameters between two environments were discussed. Results show that the proportion of pores with size ≥100 nm has the best correlation with chloride diffusivity of concrete. Besides, chloride diffusion coefficients and main microstructural parameters both decrease with exposure time. The decreasing rate increases with the increase of FA content. Moreover, in the laboratory environment with high temperature and high salinity, an acceleration effect on chloride diffusion can be observed. Results of similarity analysis indicate that chloride diffusivity and main microstructural parameters at the exposure time of 320 d in the laboratory environment is the most similar to that in site at the exposure time of 840 d. For concrete with FA content not more than 40%, their Jaccard coefficients of similar criterion parameters related to chloride diffusivity and microstructural parameters are all greater than 0.87.