Abstract: In the plate thickness of 6-16 mm butt weld testing, ultrasonic testing has certain difficulties for the following reasons. Firstly, because the sheet weld inspection area was in the near field range, ultrasonic sound pressure changes not as regularly as in the far field zone, resulting in a larger error prone for the defect location and quantity. Secondly, the sound path of sheet metal weld scanning process is shorter, the instrument connected with the probe after the match was subjected with more clutter and poor signal to noise ratio, especially in the weld caused by the reflected waves over the height, sometimes higher than rejection line(RL), easily leading to false conclusion and miscarriage of justice. Therefore, we should make use of sub-DAC curve for detection of multiple reflection method, ultrasonic detector as simulated actual use of sub-DAC curve is extremely inconvenient, and the digital machine double gate method can better solve the technical problems..