Abstract: The chloride diffusivity in cement-based composite materials is affected by multi-scale pores, including gel pores, capillary pores, entrained and entrapped voids, micro-cracks, etc. The pore-structure parameters (e.g. porosity) will change when subjected to external loadings, resulting in the change of the chloride diffusivity in cement paste. The effect of the external loadings on the chloride diffusivity can be assumed as the change of porosity on the chloride diffusivity induced by external loadings. In the present study, saturated cement paste is regarded as a two-phase composite composed of instinct cement matrix (with zero porosity) and pore-water inclusion. Based on the theory of elasticity, the quantitative relationship between current porosity of mortar and initial porosity as well as volumetric strain before reaching the strength of mortar (i.e. before the appearance of new cracks) is deduced. Moreover, the quantitative relationship between the chloride diffusion coefficient of cement paste and these parameters is obtained. The effects of external loadings (herein i.e. the volumetric strain) and porosity variation on the diffusion behavior of cement paste are investigated on the basis of the Fick’s second law. It is found that the chloride diffusion coefficient in saturated cement paste increases significantly with the increasing of initial porosity. Furthermore, the diffusion coefficient decreases with the increase of compressive strain, while the diffusion coefficient increases with the increase of tensile strain.