In order to detect weak ultrasonic signal,a high-sensitivity optical heterodyne interferometer was developed.A linearly polarized laser with good coherence was used as light source to improve interference intensity,and a diaphragm was employed to eliminate the optic noises and the influences of feedback lights on the light path.Then,the system was easily adjusted by using the 1 st order light as reference light.Experimental results indicate that the measured vibration signal frequency and amplitude are 62.38 kHz and 76.4 mV respectively,when a square wave is adopted to excite the ultrasonic probe with 62.42 kHz central frequency and 74 V amplitude,which shows proposed system can satisfy the detection requirement of weak ultrasonic signal.