Momentary flywheel speed is the most convenient signal to detect cylinder-by-cylinder variations. Though some papers had indicated that torsional vibration of crankshaft should be considered at high-speed condition , it is still not very clear how to quantitatively analyze the influence of the vibration upon the flywheel speed. The goal of this paper is to reveal the influence and to find a way to compensate it. At first the transform functions of each cylinder torque to flywheel speed are analyzed. Then based on the transform functions, rigid crankshaft model and flexible crankshaft model are compared. The influence of the torsional vibration on the flywheel speed is investigated. Finally, a compensator of the influence is built. The compensator is verified by the simulation results. After compensation, the measured flywheel speed can be processed under the rigid crankshaft model.