Abstract: In this paper, the phased array ultrasonic testing technology for the welding seam of heat exchange pipe cap of synthetic reactor in a chemical plant is analyzed and studied. The developed special scanning device and wedge are used to perform the test on a contrast block and to optimize the testing technology. The optimized testing technology is verified on the simulation block. The results show that the optimized detection process has a good detection effect, which provides an effective solution for the detection of pipe cap weld.