This article presents a novel rope-driven antagonistic variable stiffness robot elbow joint based on permanent magnet spring and pulley block. The proposed joint enlarges both the range of motion and stiffness. The structure and working principles of the elbow joint are elucidated, along with the joint stiffness model. The changing pattern of joint stiffness is analyzed, and a controller is devised to decouple the stiffness and position of the joint. Experimental results verify the accuracy of decoupling and showcase the energy-saving characteristics of the joint. Furthermore, the paper investigates the impact of joint stiffness variation on joint position control.