Aiming at the variable stiffness drive function requirement of robot joint, a variable stiffness drive method based on magnetorheological principle is proposed. The method utilizes the characteristics that the rotary magnetorheological damper torque is controllable and the response speed is fast, and the rotary magnetorheological damper and servo motor are combined to form a variable stiffness driver. Based on the configuration of the variable stiffness driver, the principle of variable stiffness drive was analysed. The design of the rotary magnetorheological damper used for variable stiffness driver was completed, the torque value and the magnetomotive force value of the damper were obtained. Based on the configuration of the variable stiffness driver and the mechanical characteristics of the damper, the variable stiffness control process was designed and the control program was developed. On this basis, the test system of rotary magnetorheological damper and its mechanical performance was developed, the actual mechanical performance parameters of the damper were obtained, the maximum torque of damper was 18. 1 N·m. The variable stiffness driver test system was established and the experiment research was completed, when the stiffness coefficient was set as 5. 0 N·m/ rad,6. 0 N·m/ rad,7. 0 N·m/ rad respectively, the actual stiffness coefficient obtained was 5. 1 N·m/ rad,6. 2 N·m/ rad,7. 1 N·m/ rad corresponding. The experiment results show that the proposed method can realize the stiffness adjustment of the driver nicely.