Abstract: For an upper limb rehabilitation exoskeleton, two passive human-machine gravity balance strategies were proposed to decrease the joint torque and the power of the driving motor. Two units of zero-free-length springs were added in the exact gravity balance system of the exoskeleton. According to the constant potential energy of the human-machine system, a balance model of the exact gravity balance system was established. To simplify the balancing system and avoid colliding with human body, an approximate gravity balance system was presented and only one unit of zero-free-length springs was introduced in the exoskeleton. The locations and main parameters of the installed springs were optimized and obtained based on the minimum change of the total potential energy in the given workspace. To verify the balance system mentioned above, two examples were compared and analyzed by calculating the joint torques at different poses of the exoskeleton. Results show that the torque of each joint can be eliminated completely by the exact gravity balance system and reduced mostly by the approximate one.