This paper investigated the pore structure of fly ash with the object of undisturbed fly ash samples prepared by freeze drying. By incorporating microscopic digital imaging technology and professional image processing technology, the porosity density dimension of fly ash was derived on the basis of the fractal theory, and the cause of the multi-fractal property of pore structures of fly ash was analyzed. A function quantifying the relation between the permeability and the porosity density dimension of the fly ash was derived by employing capillary bundle model and Poiseuille equation. Parametric analysis indicates that the permeability of fly ash increases with the increase of the maximum pore radius and the inflection point of pore radius, and decreases with the increase of the fractal dimension of the pore radius distribution in sections I and Ⅱ.