The structure and principle of cone connection without key in diaphragm coupling are introduced.The position where the maximum stress located in and the relation between prestressing force and the corresponding torque in cone connection with the analysis element method is analysized.The analytic method and finite element method about the pre-tightening force are compared when transferred the same torque.The finite element method is more intuitive and accurate compared with the analytic method and could be used as a new method during the designing and manufacturing process.