The helicopter tail drive system was simplified to a series system with shaft section and equivalent disk.A equivalent dynamic analysis model with multiple degrees of freedom,in which the transmission error of meshing gear pair and meshing stiffness of tail-decelerated gear were included,was established for this simplified helicopter tail drive system.According to the torsional dynamics equation of helicopter tail drive system,the torsional vibration response of system was obtained, and the torsional vibration characteristics of the system with different torsional stiffness of the shaft and meshing stiffness of the gear were discussed. The following conclusions may be drawn from the analysis result. The torsional displacement of tail rotor is larger than the torsional displacement of driven gear of tail gearbox, namely the vibration response of output gear of tail gear box is larger than the input gear. The variation tendency is opposite of the torsional angle displacement between the equivalent disk connected to the horizontal shaft and oblique shaft respectively. The meshing stiffness has more influence on the system than the shaft torsional stiffness, it should be taken account into the modeling of helicopter tail drive system.