Abstract: The main difficulties of the volume inspection for thick wall HDPE pipe hot melt joint were analyzed. Based on the acoustic properties of HDPE material and the acoustic field modeling analysis of phased array ultrasonic, self-focusing phased array ultrasonic probe and special structure water wedge are customized, which effectively solves the ultrasonic detection problem of HDPE hot melt joint with wall thickness greater than 90 mm. The effectiveness of the detection technology was verified by the field inspection of 32 inch HDPE pipeline of the water system in a nuclear power plant.