According to the mechanism of the interferometric fiber-optic sensor, an optical orthogonal heterodyne demodulation technology has been investigated in this paper. A variable fiber delay line was used to adjust the optical phase delay to obtain optical orthogonal heterodyne demodulation, so that the low-frequency noise in the photo-detector can be reduced. Comparing with the conventional heterodyne demodulation schemes, the additional noise as well as temperature-related drift induced by the electronic phase drift devices can be eliminated by using the proposed demodulation technology. An experiment system was set up to measure the optical phase change induced by the piezoelectric ceramic (PZT) vibration. The result shows that the proposed scheme can demodulate the sensing signalefficiently, and a minimum detectable phase shift of 9.2×10-5 rad can be achieved.