The properties of clay, such as rheology, expansion and penetration, were affected by clay bound water ,and it was important to understand clay bound water for the study of clay. Based on diffuse double layer theory of clay, the relationship between the initial shear stress of clay bound water and the distance from the water to clay particles was derived in the micro electric field of clay bound water, Which was used to explain the mechanism of the non-Darcy flow of clay. The results show that clay bound water has proper Non-Newton liquid properties. There is an initial shear force in each location of bound water. When the water pressure exceeds the initial shear force, the flow will occur. There is an initial hydraulic gradient in non-clay Darcy flow. When the flow occurred, there is a linear relation between the initial hydraulic gradient and the initial shear stress. When the flow occurred, the distribution of bound water flow velocity appears curve and there is a nonlinear relation between flow and the initial hydraulic gradient. The percolation flow curve of clay appears downward concave curve.