A variable-stiffness mechanism using permanent magnet spring was proposed for elastic robots. Due to the motor torque limit, in the case of no increasing in wire tension, strong variable-stiffness was achieved. The permanent magnet variable stiffness mechanism was mainly composed of permanent magnetic spring structure and wire-driven system. The permanent magnetic spring structure was composed of a pair of annular permanent magnets which were arranged in coaxial and the same permanent poles were opposite. The wire-driven system was composed of a movable pulley and two fixed pulleys. The magnetic force of permanent magnetic spring and wire tension of wire-driven system mathematical models were established by virtual displacement method. The magnetic force and wire tension were also measured by experiment. The results of experimental measurement were in good agreement with the calculated results. The calculated and experiment results together showed that the magnetic force, wire tension and stiffness were increased nonlinearly with the decrease of air gap between the permanent magnets, and increased with the increase of axial length and average radius of the permanent magnet. The height of triangle structure of wire-driven system was kept constant, the wire tension and stiffness were increased nonlinearly with the increase of the triangle structure bottom of wire-driven system. Decreasing the radius of the pulley can further increase the range of the wire stiffness.