Abstract: In order to measure the actual elongation of piezoelectric transducer(PZT) driven by fast alternative electric field, a method based on frequency-modulation(FM) laser heterodyne signal demodulation technique is proposed. The method shifts the frequency spectrum of signal by using FM laser heterodyne technique so that the signal will not be disturbed by low-frequency noises. In that case, the Fourier spectrum of end plane velocity of PZT is measured accurately, with which the velocity variation can be deduced. Then, the elongation of PZT can be obtained by performing integration on the velocity. The results show that, this method simple, can acquire high quality signal easily, and has high sensitivity high accuracy.