The permeability coefficient of sand soil can be reduced by mixing mud，bentonite，and cement，meanwhile the mix proportion meeting the permeability requirements of the sealing wall can be obtained by the indoor mix proportion test. Based on the theory of average permeability coefficient of layered soil，the theory of permeability coefficient of the sand sealing wall is established. The permeability coefficient of the mixed soil is controlled by the permeability coefficient and thickness of the fine-grained soil. The permeability coefficient of the mixture is mainly changed by the water absorption and expansion of the bentonite，and the hydration reaction between cement and pore water in the soil mainly changes the permeability coefficient of the fine-grained soil. The results show that the thickness of fine-grained soil，the permeability coefficient of the mixture with bentonite，and the permeability coefficient of fine-grained soil mixed with cement can be calculated by the theory of permeability coefficient of sand sealing wall.