Abstract: The rock structure planes with low to middle inclination angles at some dam regions in the southwest of china have the spatial orientation rule of tendency toward either upstream or downstream. According to the structure of the surface contact, the planes are first divided into two big kinds: the rigid structure and the weak structure. Among them, the rigid structure belongs to the crevasse. The weak structure surface, according to its material composition and the structure characteristic, may be subdivided into 3 sub-groups including the clod rock debris, the rock debris clamps mud, the mud clamps the rock debris. Along the direction perpendicular to the fault strikes and from high elevation to low elevation, the fault structural planes have a development pattern gradually changing from the mud clamps rock debris, to the rock debris clamps mud, to the clod rock debris and to? crevasse . Finally based on the analyzing of shear test data, classification and development pattern, this paper puts forward an index value of shearing strength.