Abstract: The weak structural plane is an important engineering-geological problem in construction of hydroelectric project. To choose rational mechanical parameters is of great significance for determining the dam foundation surface and the stability against sliding. In this paper, the correlation between granulometric composition and shear strength parameter weak interlayer in sedimentary rocks is studied, and the correlation equations are established for different consistency states. Finally, in combination with quantitative indexes determinied in the code for Design of Concrete Gravity Dams, the shear strength parameters of weak structural plane are determinied.