To address the difficulty in detecting defects in butt fusion joints of high-density polyethylene (HDPE) gas pipelines, this paper proposes a microwave imaging method based on the molecular chain structure characteristics of cold welding and over-welding defects, as well as microwave detection principles. This method is used to detect and image typical defects (cold welding and over-welding) in HDPE gas pipeline butt fusion samples. The results show that there are differences in dielectric constant between cold welding and over-welding defects, which can be well reflected in the changes of microwave reflection coefficient amplitude. The average microwave reflection coefficient amplitude of the butt fusion joint of a defect-free sample is around -34 dB, while that of the butt fusion joint with a cold welding defect is around -31 dB, and that of the butt fusion joint with an over-welding defect is around -35 dB. The microwave reflection coefficient amplitude increases as the heat absorption time, welding temperature, and welding pressure decrease. By correlating the areas with abnormal microwave reflection coefficient amplitude with the specific locations of sample defects, and comparing the results with digital radiographic (DR) detection, it is confirmed that this microwave imaging method has better detection capability for cold welding and over-welding defects in HDPE gas pipeline butt fusion joints.