Abstract: Based on the torsional motion equation of shafts, the solution of torsional vibration of a tail drive system, whose transmission ratio of an intermediate gearbox and a tail rotor gearbox is not equal to 1, is obtained by the superposition principle of vibration modes. Then, the effect of transmission ratio on torsional vibration of a helicopter tail drive system is analyzed. Results show that when transmission ratio of an intermediate gearbox or a tail rotor gearbox is slightly larger than 1, the maximum amplitude of torsional response of an intermediate drive shaft and a tail drive shaft is larger compared with the case of transmission ratio beging 1. The second torsional frequncy of a helicopter tail drive system increases, the third changes less, the frequencies of modes higher than three change little.