Abstract: The micropore structure characteristics have significant influence on permeability of the soil, and bentonite is high anti-seepage material, which is added into the soil to reduce the peamiability by changing the pore-scale distrbution and pore characteristics of the soil. The micropore structure characteristics of the soil added with different bentonite contents are measured by the mercury intrusion porosimetry (MIP). The test results show that just adding a little bentonite can reduce the quantity of the large pore abruptly. With the increase of bentonite content, the pore-scale distribution changes gradually, the quantity of the middle pore increases, the quantities of the small pore, the micropore and the supermicro-pore increase gradually, and the pore size distribution density function changes from three-model to bi-model while the pore diameter size corresponding to the peak decreases. It is proved that the pore characteristics of the same void ratio are various according to the test results, so the void ratio which is a macroscope parameter can not represent the pore characteristics of the soil perfectly and it is necessary to introduce other microscope parameters to describe the pore characteristics of the soil. The conclusions have certain referential value for the design and construction of the anti-seepage (infiltration) projects.