2, there are many types of flaw detectors, but in the actual flaw detection process, pulse reflection ultrasonic flaw detector is most widely used. Generally in the uniform material, the presence of defects will cause material discontinuity, this discontinuity often causes acoustic impedance inconsistency, by the reflection theorem, we know that ultrasonic waves in two different acoustic impedance of the media interface will occur on the reflection, the size of the reflected energy back to the interface on both sides of the media acoustic impedance difference and the interface of the orientation, the size of the relevant. Pulse reflective ultrasonic flaw detector is designed according to this principle.