Based on the rapid determination of chloride diffusion coefficient with NEL method and pore structure analysis, the influence of HCSA expansive agent on chloride diffusion coefficient of concrete has been investigated. Besides, the effects of chloride diffusion coefficient of Water-Cement ratio on HCSA expansive concrete have been also analyzed. The results show that on the condition of 55% mixing amount of fly ash, the chloride diffusion coefficient of concrete of no HCSA is 1.96×10－8cm2/s, which is classified moderate. While the concrete mixing HCSA is about 0.62×10－8cm2/s, the permeability belongs to inferior. The effects of permeability of HCSA mixing amount on concrete is not obviously. The Water-Cement ratio affects the concrete permeability. Under the experimental conditions, the water-cement ratio of 0.4 can achieve better effect of the HCSA expansion; Furthermore, the mercury intrusion further verified that incorporating proper amount of HCSA can make the internal structure dense and improve the density and impermeability of concrete.