The variable permanent magnet spring joint made up of the permanent magnet spring has good nonlinear stiffness change performance, which can improve the safety of human and robot interaction. To further improve the range of motion and stiffness performance of the robot, based on the principle of magnetic flux superposition, a type of robot variablestiffness joint was proposed, which increased the adjustment speed of the joint stiffness and motion range of the joint, while reducing the joint mass. The variable stiffness working principle of adjustment of magnetic flux in permanent magnet-electromagnetic hybrid variable stiffness device by electrical signal was elaborated, and the overall mechanism of the joint was designed based on this principle. The joint stiffness model was established according to the magnetic flux continuity principle and the virtual displacement method. And the relationship between the joint stiffness with the current and the joint position was given. Based on the variable-stiffness joint stiffness model, the position and stiffness coordination controller of the variable-stiffness joint was designed, and the principle prototype of the variable-stiffness joint was built. The proposed variable stiffness robot joint based on the magnetic flux superposition principle had lighter weight and better structural stiffness change performance and motion accuracy. The overall joint mass was 1.1kg and the joint can rotate from -180° to 180°. The experimental test results showed that the variable stiffness robot joint based on the superposition principle of magnetic flux can realize rapid adjustment of joint stiffness, and the adjustment time was only 0.133s. The position and trajectory tracking accuracy of the joint was increased with the increase of stiffness and decreased with the increase of frequency.