Abstract: With the growth of physical human-robot interface, the security of human-robot interface has attracted attention recently. Mechanically musculoskeletal variable-stiffness mechanisms applied to robot joints can solve the problems about human-robot security and unknown environmental adaptability by imitating the variable stiffness property of skeletal muscle. Many mechanism designs of variable stiffness joints of overseas robots are studied. The nonlinear variable stiffness principle of mechanically musculoskeletal variable-stiffness mechanisms is summarized. Mechanism design methods are classified into five categories, and their advantages and disadvantages are analyzed. The analysis results might provide references for designs of highly-safe and well-adapted robots.