This paper discusses a space manipulator for grasping non-cooperative objects and an envelope grasping planning method for objects of different shapes and sizes. A novel space deployable manipulator is proposed in this paper, which consist of a finger mechanism and a wrist mechanism. Then, a novel envelope planning method is proposed to make the manipulator grasp objects of different sizes more efficiently and stably. The static analysis of grasping process is carried out, the factors affecting grasping balance are analyzed, and the mathematical model of grasping balance is established. An envelope grasping optimization model with the driving rope length as variable is proposed to obtain the best grasping posture. Finally, the envelope planning simulation is carried out, which verifies that the novel manipulator proposed in this paper has good shape adaptability and size adaptability, and verifies that the envelope grasping method is feasible.