A tandem tensioning constant-torque mechanism is designed. A constant-torque system is used to absorb the response lag of motor under the high-frequency impact and keep the constant tension of the string passively. In the bypass tensioning constant-force mechanism, the friction problem exists because of sling's compression deformation on the pulley. To avoid this kind of energy loss caused by friction, the tandem tensioning constant-torque mechanism without pulley is selected. From the view of working-energy storage, the constant-torque mechanism is an energy storing mechanism with constant proportion of energy storage-angle. Based on this viewpoint, both pressure spring-cam-torsion bar and pressure spring-torsion bar constant-torque mechanisms are proposed. Then the principle analysis and theoretical validation are performed respectively. The mechanism design, simulation and experimental validation are conducted on the pressure spring-torsion bar constant-torque mechanism with smaller friction and moment of inertia after the comprehensive comparison. This simple mechanism takes up the spatial size of 1000 mm × 350 mm × 350 mm. The experiments prove that the mechanism's equivalent stiffness is 1.47 N/mm, adjusting range of the static working point is 0～ 1000 N, and the positive and negative error is ± 15 N.