This paper focuses on the severe torque vibration phenomenon during start up process of an Integrated Starter Generator (ISG) assisted hybrid powertrain, which may bring damage to the connecting shaft and be a great threat to driving comfort. In this paper, the shaft character between torsional vibration damper and ISG motor is investigated and the whole mechanical components of powertrain are modeled with the help of a commercial simulation platform AMESim. Besides, an experiment is designed and conducted by dragging the engine to normal operation with a constant torque provided by ISG, which ensures a single excitation source and can bring an in-depth perspective towards the torque vibration caused by the engine compression strokes. The toque vibration can be recorded by a high frequency torque sensor while the motor speed and torque signal can be obtained from motor control unit (MCU) by CAN. Furthermore, the parameters of the powertrain model are identified by genetic algorithm on the basis of experimental data. The simulation results are compared with that recorded by the torque sensor through short time Fourier transform (STFT) numerical analysis. Results show that both the simulated and tested torsional vibration frequency can coordinate well with the spark frequency of the engine.