The shear behavior of joints was not easy to predict under constant normal stiffness (CNS) conditions. The normal compressive stress would increase during shearing when the joints dilation took place. Meanwhile the increasing of compressive stress would cause the restriction of dilation

which would finally reduce the increment of compressive stress. A simple model was presented to simulate the joints shear behavior under CNS conditions. It was based on the relationship between normal displacement and shear displacement of the joints in the shearing process

from which the normal compressive stresses in the CNS conditions were calculated iteratively and the shear stresses were obtained finally. The influences of parameters of rock joints on shear behavior under CNS conditions were also discussed. The results show that the shear stresses are determined coordinately by the constant normal stiffness and the normal deformation parameters of joints

including the uniaxial compressive deformation and the decreasing of dilation angle; and the shear stress will reach a high value if the joints are not easy to compress during CNS shearing process.