Abstract: The helicopter tail drive system was simplified,and it was presented that a helicopter tail drive system was a cascade system comprising shaft segments and disks.A bending dynamic analysis model in which the support stiffness of the bearing and coupling were included was established.Using transfer matrix method,the differential equations were solved.The results show that the effect of the angular stiffness and the radial stiffness of coupling on natural frequencies of the helicopter tail drive system is different,and the natural frequencies could be reduced with the increase of shaft segments or stiffness of the bearing.