Abstract: A modulated laser interferometer used for piezoelectric characteristic measurement of the piezoelectric ceramic is discussed. By this method, it can not only suppress the effect of noises on the interference signal, but also adequately reduce the effect of so called 1/f noise of the photo-detector and electric elements. Compared with general interferometric method, the modulation interferometry has an advantage of high precision. Finally, the piezoelectric constant d31 of a piezoelectric ceramic versus the frequency and amplitude of the excited voltages,and the hysteresis loop of the material are measured with this method.