In order to solve the failure of an axial flow induced draft fan after the variable frequency driven conversion, the torsional stress pulsation characteristics of the fan were tested by using the wireless strain measurement technology. It was found that the torsional stress fluctuation amplitude increased sharply when the fan was running over a certain rotating speed and then kept high. In this process, the torsional stress pulsation frequency was locked and did not change with the rotation speed, which was different from the phenomenon of torsional resonance. The results showed that the unstable torsional vibration occurred in the electromechanical coupling system composed of frequency converter, motor, shaft and fan. It was proposed to solve the unstable torsional vibration problem by replacing the diaphragm coupler with the high elastic rubber coupler. The experimental results before and after replacement showed that the high elastic coupler could effectively suppress the unstable torsional vibration.