Abstract: This article proposes a new diagnostic method based on ultrasonic testing to address the difficulty of diagnosing thermal defects in 110～220 kV high-voltage cable joints. Defect pre positioning is achieved through analysis of ultrasonic propagation characteristics, and precise positioning and imaging are carried out using tomographic imaging technology, combined with feature parameter extraction for quantitative evaluation. The experimental results show that this method can effectively identify the thermal damage state of cable joints. The sound velocity of the thermal damage joint decreases to 2085 m/s, the reflected wave amplitude increases to -21.3 dB, and the attenuation coefficient reaches 2.23 dB/(mm·MHz), providing a reliable basis for evaluating the degree of degradation of cable joints.