Abstract: This paper intends to develop a novel planar hybridly actuated two-finger gripper with three phalanxes, namely a kind of gripper whose intermediate and distal phalanxes are underactuated through cable-pulley, and proximal phalanx is driven by a separate motor otherwise. Therefore, a dynamic analysis method based on virtual prototype technology and MATLAB is proposed for the underactuated gripper to complete optimal structural design. Firstly, the gripper model is simplified for dynamic analysis, and the structure design variables of the gripper are determined. Secondly, three kinds of performance indexes are defined for grasp quality, including stability, flexibility and adaptability. Moreover, a comprehensive evaluation index is proposed for the underactuated gripper in the cases of different driving forces, different grasped objects as well as different initial poses through combining different mechanical design parameters and carrying out dynamics simulation in a batch process. Thus, an optimal objective function with weights is designed for different grasping cases. Finally, a structure scheme of underactuated gripper suitable for expansion of the multi-finger hand is designed according to the requirements of outdoor sampling mission and the simulation results, in which the distance between the modular fingers is adjustable.