Numerical simulation was performed on chloride diffusivity in non saturated hardened cement paste, in which saturation function was related to saturation degree. In particular, the solid mass fractal model was applied to simulate pore structure of hardened cement paste. Meanwhile, the lattice Boltzmann method was applied to simulate chloride diffusion. The variation of saturation degree was addressed in terms of the successive filling of pores with respect to pore size distribution. The present numerical results agreed well with the results calculated from classical power type saturation function, and the saturation coefficient should be of 45.