Abstract: To realize the functions of clamping force adjustment and target object pulling, an underactuated manipulator is designed based on planetary gear mechanism. Because the output shaft and ring gear of the planetary gear mechanism are not fixed, the underactuated manipulator can be assigned two mutually exclusive motions. One motion is used for the rotation of the manipulator fingers to realize the gripping function of the manipulator. The other is used for the infinite rotation of the pulley, and the pulling function of the target object is realized through the belt. Relative to the tendon rope and link type underactuated manipulators, this single-input double-output mode can not only maintain the adaptability of the manipulator, but also reduce the coupling degree of the manipulator. According to the transmission characteristics of the planetary gear mechanism, the resistance regulation mechanism is designed, which realizes the function of adjusting the gripping force of the manipulator. The clamping force test results show that the manipulator can effectively adjust the gripping force under the adjustment of the resistance regulation mechanism. The grasping experiment results show that the manipulator can grasp the rigid and flexible objects, which verifies the effectiveness of the manipulator.