Combining with the design and development of circumferential short spring automobile dual mass flywheel, the safety protection device based on auto-lock theory is proposed. This device can automatically connect the primary and secondary flywheel together by wedging the profiles of them with the auto-lock feature to avoid driving accident when flywheel fails, and this device is reliable and with simple mechanism. By establishing the force analysis model considering friction, the equation of ensuring auto-lock shape constraint profile is obtained. Based on the above and with the application of Winkler elastic theory, the analysis calculation model of contact force and torque-torsion angle based on shape constraint is established. With the application of the theoretical method, the safety protection device of the dual mass flywheel assembled on the 2.0 L engine is designed. Results of finite element analysis and real test verify the theoretical method and the calculation model proposed comply with the reality. By thorough analysis and research, design criteria of safety protection device of dual mass flywheel is summarized and the proposed design theory and method can be applied to design practice, which broadens the design theory of dual mass flywheel.