With the application of High-density polyethylene (HDPE) pipe with thick wall in nuclear power plant (NPP), great attention has been paid to the safety of the pipeline’s joints, which can be assessed by phased array ultrasonic testing (PAUT). PAUT creates constructive interference of acoustic waves to generate focused beams according to delay law based on time-of-flight. However, due to the existence of acoustic attenuation and dispersion, waveform distortion occurs when ultrasonic pulse propagates in HDPE, which will accumulate with the increase of propagation distance, and then results in imaging errors. In this paper, the relationship of acoustic attenuation and dispersion in HDPE was obtained by numerical simulation in Field II®, which can be verified by the experiment of our previous work. Besides, the investigation of the waveform distortion revealed the linear relation between peak offset and propagation distance. Considering the relation, an improved delay law was proposed to increase the intensity of ultrasonic field. This improved delay law was compared with the conventional one by numerical simulation of ultrasonic field and PAUT experiments, which showed that the improved delay law could increase the image sensitivity.

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