Laser ultrasonic signal is so weak that it can be detected only by high sensitivity system. A laser heterodyne interferometer system is designed, in which polarized laser with good coherence is used as light source. A diaphragm is used to eliminate the optic noises and the influences of the feedback lights on the light path. The system is easy to be adjusted by using the 1 class light as reference. Experimental result shows that a good heterodyne signal is obtained by the system.