Abstract: Considering the specificity of thin metal plate, an ultrasonic phased array wedge is developed by computing and analyzing with Snell’s law. By means of sound field simulation and actual test, the effect of the wedge for testing defect in friction stir welds is proved. It proves that with this method the design of wedge can be optimized and can get rid of blindness. The wedge made according to the design shows great effect, which can detect the lateral hole locating 1mm under the upper surface whose diameter is 0.3 mm in the 3 mm plate.