The shaft torsional vibration model of an axial flow fan with variable frequency drive(VFD)was set up for the analysis of shaft break fault. The mechanical/electrical interaction effect of the fan with the vector convertor speed governing technique was considered. The variation of the driven shaft instant angular speed and torque during a run up process was simulated. The pulsation torque spectrum was given. The torsional vibration and the pulsation torque were measured in the field.It is shown that abundant harmonic components exist in the three phase signals output from the frequency converter. The harmonic components may coincide with the shaft torsional natural frequency in certain speed zone, which causes large torsional vibration and torque pulsation. In the high rotating speed zone, the selfexcited torsional vibration caused by the mechanical/electrical interaction effect may occur.The torsional amplitude of the shaft sections beside the both ends of the coupling is different due to the difference between the moments of inertia of the fan and motor. It causes high torque pulsation on the coupling and therefore the break of the shaft and coupling, which is of great influence on the safe and reliable operation of the fan. So, the torsional dynamics and shaft strength should be analyzed before the VFD modification.