Aiming at a seriesparallel hybrid powertrain system which contains engine, motor and planetary gear subsystems, this paper considered a typical working condition which is based on the power control strategy and established the torsional vibration mechanical model of a hybrid powertrain system. The inherent characteristics and transient vibration response of the electric mode, hybrid mode and parking charging mode were studied. It was discovered that the repetitive frequency of the powertrain system in the three working modes is the same which is only related to the inertia and meshing stiffness of the planetary gear system. The nonrepetitive frequency and corresponding vibration modes in the electric mode and parking charging mode are almost identical. The transient response of body acceleration has the same frequency component as the interference torque of the excitation sources, the body vibration caused by torque ripple become obvious when the starter drives the engine and stays in the regenerative braking mode at low engine speed. The angular acceleration of the planetary gear system significantly increases due to the torque fluctuations caused by the working mode switches.