To address the issue of low transmission torque in radial magnetic coupling variable stiffness joints, a variable stiffness joint based on axial magnetic coupling is designed. Firstly, based on a simplified magnetic pole unit model, an analytical formula for magnetic coupling torque was established to conduct the preliminary design of permanent magnet dimensions. Then, a three-dimensional simulation model of the permanent magnet and the yoke iron was established using Maxwell software, and the torque and axial magnetic force characteristics were analyzed by scanning load angle, air gap thickness, and changing the number of magnetic pole pairs. Finally, a variable stiffness joint prototype was fabricated using permanent magnets with appropriate structural parameters, and an experimental system was established. The torque and variable stiffness performance of the prototype were tested to validate the effectiveness of its design. The results showed that when the air gap thickness was less than 10 mm, the theoretical model and the simulation model were relatively consistent in the torque prediction, with a maximum relative error of 11.6%, while the maximum relative error of axial magnetic force was 13.7%. Increasing the number of magnetic pole pairs could significantly enhance the joint torque, but it would reduce the rotation range of the joint. In addition, due to the increase in leakage flux, the decay rate of the joint torque became faster as the air gap thickness increased. Experimental tests showed that the designed variable stiffness joint prototype achieved a torque transmission of 6.43 N·m and an active stiffness adjustment range exceeding five-fold, with a relative error between the actual operating data and the simulation results within 7.8%. The proposed method can provide theoretical support for the optimization design of axial magnetic coupling variable stiffness joints.