Abstract: A dynamic simulation model was established using AMESim for investigating the torsional vibration in the drive line of a series-parallel hybrid electric city transit bus. The torsional vibration analysis was conducted to understand the influence of main factors. Based on typical city bus operating characteristics, operating procedures were constructed, in which the hybrid powertrain experienced vehicle launching, low-speed running, engine start, high-speed running and regenerating braking. Simulations were performed to understand the influence of key component design parameters or a dynamic duty cycle. Simulation results show that the torsional vibration responses are closely related to spring stiffness and damping coefficient of the input damper and clutch damper, as well as the degree of speed synchronization upon clutch engagement. The results also reveal that the torsional vibration of a drive line is effectively reduced by reducing the ISG motor torque for starting the engine, and by limiting the acceleration of the hybrid powertrain when a rapid acceleration or a deceleration happens.