Cable winch with constant tension function

The invention discloses a cable winch with a constant tension function. The cable winch comprises a cable roller, a supporting frame, a speed reducer and a motor; the cable roller is fixedly provided with a rotating shaft; two stuffing boxes are arranged on the cable roller; external cable ports of the two stuffing boxes face opposite directions; a slip ring box with a conductive slip ring is fixedly arranged on the supporting frame; the two stuffing boxes are correspondingly connected with the two conductive slip rings in the slip ring box through slip ring connecting cables; an inner cavity of the cable roller and an inner cavity of the slip ring box communicate with an inner cavity of the rotating shaft; the slip ring connecting cable penetrates through the inner cavity of the cable roller, the inner cavity of the rotating shaft and the inner cavity of the slip ring box; a dynamic torque sensor is arranged on an output shaft of the speed reducer; the dynamic torque sensor is connected with a PID controller