Abstract: The shear strength mechanisms of joint and rock bridge are analyzed in this paper. Though introducing deformation compatibility equation in normal direction, the direct shear strength formulation is deducted based on the Mohr-Coulomb theory. According to the model test, tensile-shear complex failure is the main failure mode on the shear plane. Many tensile micro-cracks are formed in the test sample. There are five typical stages in the failure process. They are the linear elastic increment, the slippage of the joints, the steady development of post-cracking, the coalescence failure of discontinuous joints and the residual strength. The theoretical calculated results coincide well with experimental results.