This paper relies on the thought of block theory to establish appropriate parameter matrix and figure out its discrimination matrix in view of the fact that the block in face-contacted block structure has high intensity and stiffness, the friction face among blocks approximates a plane, the structural face runs through the block and structural instability is mostly due to shear slip of the block along the contact face. Vector analysis method is employed to identify the mobile block in face-contacted block structure. And as a further step, the paper probes into specific identification method for the key contact face in face-contacted block structure by means of virtual tangential “net slipping force” F. This lays a solid foundation for further exploration of face-contacted block structure.