Chloride diffusion coefficient is one of the important parameters to study the durability of concrete structures in marine environment. Through mercury intrusion test and salt spray diffusion test of concrete with different water-binder ratios, the influences of porosity, pore size distribution and critical pore size on chloride diffusion coefficient in concrete were studied. The relationship between the fractal dimension of pore volume and the chloride diffusion coefficient was established based on Menger sponge model. The results show that the porosity and critical pore size are highly correlated with the dimensionless chloride diffusion coefficient, which can be used as important parameters to reflect the chloride diffusion performance of concrete. The distribution of fractal dimension of pore surface calculated by mathematical analysis is 2.56~3.86, and that of pore volume is 2.85~2.98. The fractal dimension of pore volume calculated by mercury intrusion test and fractal theory can be used as an index to evaluate the chloride diffusion coefficient. The chloride diffusion coefficient decreases with the increase of the fractal dimension of pore volume in four ranges of pore size less than 10 nm, 10~100 nm, 100~1 000 nm and more than 1 000 nm.