A high-sensitivity optical heterodyne interferometer system was developed and used to detect weak ultrasonic signal. Polarized laser with good coherence was used as light source to improve interference intensity. A diaphragm was used to eliminate the optic noises and the influences of the feedback lights on the light path. Besides

the system is easy to be adjusted by using the 1 class light as reference. A square wave was used to excite the ultrasonic probe with 62.42 KHz central frequency and 74V amplitude; the measured vibration signal frequency and amplitude were 62.38 KHz and 76.4mV respectively using the proposed system. Experimental results showed that the optical heterodyne interferometer system can be used to detect the weak ultrasound signal.