Abstract: To meet the requirements of mechanical dimensions, motion flexibility and working space for micro instruments in MIS (minimally invasive surgery), a modular joint driven by steel wires is designed. By studying the motion characteristics and topology method of the modular joint, a designing procedure of micro instrument is presented, in which the degree of freedom of the instrument wrist can be configurated randomly by using the modular joint. To verify the feasibility of the procedure, a separating forceps with four modular joints in its wrist is developed for MIS operations, and a normative kinematics model of the instruments is presented. The test experiments prove that the wrist of the separating forceps has good flexibility, and it can satisfy the clinical requirements for motion range and spatial flexibility for MIS operations.