Parameters including diffusion depth of chloride ion,soluble chloride ion content,pore structure of mortar and binding chloride ion capability of corresponding paste were measured under natural diffusion condition.And then factors influencing diffusion process of chloride ion in mortar were analyzed in this paper.Results indicate that the diffusion depth of chloride ion in mortar and amount of soluble chloride ion are influenced by the porosity of big pores with diameter more than 30 nm and the binding chloride ion capability of pastes.Addition of slag and chloride ion binding agent can decrease the increasing rate of diffusion depth of chloride ion with age in mortar.