Abstract: The fault zone is usually divided into fault core, broken zone and ternary structure of surrounding rock. As a water diversion structure in a large area, fault fracture zone is the key factor to induce important geological processes. The fracture zone is divided into broken rock, fault breccia and crushed powdery rock. Because the permeability coefficient of broken silt is very low, it controls the water conductivity of the whole fault zone and is a very important part of the water control structure of fault zone. Based on the typical water diversion mud structure in Gansu Province, the permeability coefficients of different structural units in the fracture zone are quantitatively distinguished by several groups of osmotic reverse pressure experiments. The results show that the farther away from the core of the fault zone, the larger the permeability coefficient is, and the variation law of the permeability coefficient is from the broken powder structure(10~9 m·s^-1). The permeability coefficient from breccia structure(10^-5~10^-7 m·s^-1)to fragmentation structure(10^-4~10^-6 m·s^-1)increases gradually, and extends to complete bedrock(<10^-9 m·s^-1). As a major characteristic of pulverized rock, low permeability is characterized by its internal fabric(particle size, pore structure, fracture structure, mineral composition, et al.), and the mechanism of low permeability of shredded rock is interpreted from microscopic aspects. The study has guiding significance for the water conductivity characteristics of the core of the fault zone and the prevention and control of water inrush in the project.