Abstract: Ultrasonic phased array technology is applied to crack detection of austenitic dissimilar steel thin-walled small diameter pipe butt welds, using special ultrasonic phased array transducer and wedge, determining test parameters by simulation, and making comparative samples for experimental research. The results show that the technique can effectively detect the artificial defects of the inner and outer wall defects of 5 mm length and 0.2 mm width and 1 mm deep, and the weld surface cracks can be accurately quantitative analysis.