Since rehabilitation robot interactively cooperates with the patients, it needs to fully ensure the safety of the user and the flexibility of their motion, and adapt to the change of interaction force in time. MRF (Magneto-rheological Fluid) is a kind of solid-liquid intelligent material with magneto-sensitive rheological property. Applying it to rehabilitation robot can design compliant joints with variable stiffness characteristics. Based on the MRF shear transmission principle, a compliant joint is proposed in this paper, which facilitates the operation compliance and intrinsic safety for rehabilitation robot. The active variable stiffness principle of MRF coupler are analyzed to support the key components of the joint. The MRF coupling stiffness model and the force-magnetic coupling model are established. The maximum torque transmitted by the coupler is tested to be 6.8 N·m. PID controller is used to adjust the excitation current when the load torque changes with square wave and sinuous wave signals. The experimental results imply that the proposed compliant joint has creditable response to dynamic load and high transmission performance.