Abstract: Studies on phased array ultrasonic testing technology for a thin-walled structure of 3 mm thick aluminum alloy friction stir welded lock welds were carried out, and the probe with 30° built-in wedge was used to solved the problems of narrow operation space and a variety of structure of the thin weld. The causes and types of defects were discussed combined with the friction stir welding process which provides a frame of reference for the similar structure weld testing and quality control.