The durability of structural LWAC exposed to chloride environments remains a subject of debate due to its porous inclusions and the unresolved transport mechanisms. This paper aims to investigate the diffusion behavior of chloride ions in LWAC by establishing a numerical model at the mesoscale. LWAC was considered a two-phase material comprising lightweight aggregates and mortar matrix. The diffusion coefficients of chloride ions for the mesoscopic components in LWAC were derived using the composite sphere model. A parametric study was conducted to further explore the chloride ion diffusion process, taking into account the aggregate shape, water-to-cement (w/c) ratio, diffusivity of aggregate, aggregate content, and grading. The results suggest that the quality of the mortar matrix had the most significant impact on the chloride ion diffusion in LWAC, while the effect of aggregate diffusivity was influenced by the w/c ratio. By incorporating exposure conditions and critical chloride concentration suggested by the existing design model, the effects of different exposure conditions on the LWAC structures were studied. LWAC structures showed shortest service life in Tidal and Splash environment, and limits on the cover thickness and the maximum w/c ratio of LWAC structures were given by the mesoscopic numerical model.