Abstract: Due to the insufficiencies of traditional underactuated manipulator in motion and function, it is difficult to grasp objects of different sizes stably. A new underactuated manipulator structure is proposed, and then the analysis and optimization of grasp are carried out. Firstly, the overall structure design of the underactuated manipulator is introduced, and then the static analysis of the finger is implemented. The structure of the finger is optimized for the instability caused by the ejection when the finger envelopes objects. Then, the reasonable size range of fingertip is determined and the optimal shape of fingertip is established based on the potential energy model of stiffness matrix. The relationship between the manipulator posture and the object sizes is expressed when the fingertips grasp objects by mathematical formula in geometric constraints. Finally, the prototype of the manipulator is built, and the fingertip grasp and envelope grasp of common household goods are carried out, which verifies that the manipulator can grasp objects of various sizes stably.