Abstract: Utilizing system matrix method,characteristics of forced vibration for the torsional vibration system of engine crankshaft are studied.Simple harmonic analysis is carried out for the excitation torque when a single-cylinder misfired.Based on these studies,with an in-line 6-cylinder crankshaft system working both under normal combustion and when facing the misfire of a single-cylinder,the system's torsional vibration characteristics under specified speed and working speed range are analyzed.The results showed that low harmonic torques mainly react on the system's torsional vibration,and torsional angle displacement increases obviously,but additional torsional stress little changes,when a single-cylinder misfires.`