To ensure the welding quality of hot-melt joints of high-density polyethylene (HDPE) pipe, this paper proposes a total focusing method algorithm based on the time reversal algorithm and instantaneous phase coherent weighting factor. The method first processes signals through the time reversal algorithm, then extracts the instantaneous phase from the signals to construct the instantaneous phase coherent weighting factor, and weights the amplitude of each pixel of the total focusing imaging. Imaging detection experiments are carried out on φ1 mm, φ2 mm, and φ3 mm transverse through-hole defects in HDPE test block and φ3 mm hole defects in HDPE pipe butt fusion joint specimens. The experimental results show that the proposed algorithm can effectively suppress background noise and artifacts, reduce the amplitude of near-field noise (0~10 mm depth) and artifacts by about 30 dB, improve the resolution of total focusing imaging. Compared with the TFM algorithm, the average signal-to-noise ratios of the four defects are improved by 9.6 dB, 11.8 dB, 4.9 dB, and 3.5 dB, respectively, and the average array performance indicators are reduced by 56%, 49%, 38%, and 48%, respectively. This provides an effective improved scheme for the ultrasonic phased array detection of defects in butt fusion joints of HDPE pipe.