The present invention belongs to the field of medical rehabilitation training equipment, and particularly discloses a reversible mechanical arm gravitational torque balancing device, comprising a counterweight guide groove module, a counterweight, a mechanical arm joint, a rope, a guide pulley block, a counterweight disc, a mechanical arm joint link and a rack. The counterweight guide groove module is mounted on the rack; the counterweight has a lower end mounted on the counterweight guide groove module and an upper end connected to the rope, and the rope is wound on the counterweight disc after passing through the guide pulley block mounted on the rack; the mechanical arm joint is mounted on the rack and internally provided with a motor; the counterweight disc is mounted on an output shaft of the motor of the mechanical arm joint; and the mechanical arm joint link is also mounted on the output shaft of the motor of the mechanical arm joint and the gravity of the mechanical arm joint link produces a gravitational torque on the mechanical arm joint. The present invention can change the direction of the provided balance torque when the mechanical arm performs the morphological transformation, and features simple, rapid and labor-saving operation as well as reliable structure.