Abstract: Ultrasonic TOFD method has advantages on weld defects testing for thick workpiece, especially high-efficiency on crack defects detection. In this paper, a USB based pocket-type testing system that combines the software part with the scan equipment was designed. It was applied to test the aluminium alloy specimen which contained different depth of defects. Furthermore, the presented algorithm could be used to automatically recognize the weld zone and linearize the depth of image instead of the original testing image. The result showed that the processed image was more convenient for defect localization by the linearized algorithm of the depth, and the testing efficiency was increased.