Abstract: To study the torsional vibration mechanism of variable frequency driven (VFD) motors, an analysis model on harmonic frequency of the shaft torque under SPWM operation mode was set up using signal analysis method, with which shaft torque pulse characteristics during run up process were tested on a VFD motor test rig. Results show that the spectrum of shaft torque pulse includes not only multiple integer harmonic components of speed frequency, but also multiple interharmonic components around the speed frequency; the amplitude of harmonic component reduces with the rise of component order; lateral vibration has slight influence on the dynamic torque, of which the amplitude increases with rising speed; in case of disturbance, many frequency components occur with higher amplitude than the case without disturbance.