Abstract: To research the torsional vibration of powertrain with the dual mass flywheel(DMF) in engine start-stop conditions, a model of DMF was built, the dynamic stiffness and loss angle in different torsional frequencies and amplitudes were obtained and the accuracy of the model was verified by tests. Combining the DMF model, an engine model was built. The analysis results show that the instantaneous big torque and torsional amplitude of powertrain will happen in the start-stop conditions and may cause the vehicle shaking and the DMF damage. The situation can be solved by increasing the damping of DMF, but it will decrease the torsional vibration damping characteristic in idle and constant speed conditions. Aiming to satisfy the different requirements of DMF damping in different conditions, combining the rheological behaviour of magneto-rheological fluid changed by magnetic field, a semi-active magneto-rheological fluid dual mass flywheel which damping could be changed was designed. The analysis results of the device indicate that the maximum torque and relative angle of magneto-rheological fluid dual mass flywheel is obviously lower than tradition DMF which could decrease the shaking of vehicle and powertrain. And the device doesn't reduce the torsional vibration damping characteristic in idle and constant speed conditions and also has a good damping effect in lager torsional shock such as suddenly speed-up, speed-down and braking conditions.