The invention discloses a constant pull device, and relates to the technical field of rehabilitation equipment. According to the constant pull device, values are set as required, constant pull is outputted, unbiased transmission is realized by the aid of a winding straightening device in the transmission process, displacement magnification can be performed on disturbance, and a feedback device performs compensation. A steel wire rope bypasses a roller outer ring, winding straightening is realized by the aid of a ball screw, a movable pulley block, a cog belt and a cog belt pulley, unbiased transmission is realized in the transmission process, precision is improved, and errors are reduced. When disturbance occurs, displacement is magnified by the aid of the steel wire rope, movable pulleys,a special-shaped lever, a pressure spring piece and a spring, and controllability and precision are simultaneously improved. When the pressure spring piece has displacement, displacement is fed backby the spring, a linear motor and a control system, so that control precision is improved, and the constant pull can be outputted.