The torsional vibration of a rotor is measured in a new way based on wavelet ridge extraction, in light of its strict requirements for the hardware of vibration extraction. First, the time-frequency information of the tooth signal is provided by the wavelet transform. Then, the instantaneous frequency is obtained from the ridge extraction of the time-frequency information, and the instantaneous speed of the rotor is derived. Finally, the parameters of torsional vibration are evaluated according to the estimation of instantaneous speed. The proposed method is verified by simulation and experiment. The results show that compared with the traditional torsional vibration testing method, the proposed method is not only more accurate but also has lower requirements for hardware. The required sampling frequency and storage space are far less than that of the traditional tooth measurement method.