The effect of the external mechanical stress on chloride diffusivity in concrete can be equivalent to the effect of porosity change on chloride diffusivity induced by external mechanical stress. And based on this viewpoint, a meso scale model and analysis method were proposed to study the chloride diffusivity in heterogeneous concrete under compressive stress. In this approach, the mechanical simulation was performed first by using the meso scale finite element method. The outcomes of the mechanical response of concrete were then used as input data for the simulation of chloride diffusivity. The concrete at the meso scale was assumed to be a three phase composite composed of aggregate, cement paste matrix and the interfacial transition zones. The meso scopic numerical model of concrete under uniaxial compressive stress was built to investigate the chloride diffusivity behavior. The effect of compressive stress on chloride diffusivity was explored, and the quantitative relationship between effective chloride diffusion coefficient in concrete and compressive stress level was obtained.