A large scale gravity compensation system for planetary rovers is designed. The compensation system comprises a tension system and a position system: the tension system generates the constant tension of the string; the position system carries the tension system to follow the horizontal motion of the rover so as to keep the string vertical. To acquire rapidity and dynamic precision without resonating the position system, a macro-micro and open-closed loop design is brought forward: the coaxial macro and micro motors drive the winding drum to adjust the length of the cable; a constant-force mechanism is used to strain the string. The macro motor generates the majority of the tension and suppresses the low-frequency disturbance; the micro motor precisely adjusts the tension of the string to suppress intermediate-frequency disturbance; the constant-force mechanism suppresses high-frequency disturbance with its open loop performance. To precisely follow the rover over a large scale, a double position servo system is used: the girder-trolley system carries the two-dimensional platform; the girder-trolley system covers a large area but is sluggish and imprecise, and therefore is used to imprecisely follow the rover over a large area; the platform is precise and fast but covers a small area, and therefore is used to precisely follow the rover. The system covers a 30m×30m area. Experiments show that the position error does not exceed 0.01m and the force compensation error does not exceed ±0.5% under normal working condition. When step interference emerges, the force compensation error does not exceed 1.5%.