| In this paper, the muddy clay that obtained from Shanghai 4th stratum has been selected to conduct a freezing thawing test, MIP test, and permeability test to figure out the freezing thawing effect on soil pore structure, especially on pore size distribution and surface fractal dimension. Combined with the fractal theory, the pores in remolded muddy clay have been divided into three categories. The results indicate that the total percentage and surface fractal dimension of medium and large pores increase after freezing thawing, with the increase of permeability. Finally, a relationship between permeability and pore structure parameters has been proposed, which consists of small pores, and medium and large pores. The permeability for small pores is several orders of magnitudes lower than that of medium and large pores.