Phased array ultrasonic technology can be used to detect lead seal defects in high-voltage cable terminals. In order to solve the problem that conventional ultrasonic quantitative methods cannot effectively detect and quantify lead seal defects, this paper proposes a quantitative detection method for lead seal defects in high-voltage cables based on ultrasonic fanscan images. The method takes the longitudinal wave fanscan image of the lead seal defect as the object and combines the threshold segmentation and corrosion algorithm to obtain other relevant information such as the cross-sectional area of the defect and the height of the defect in real time. Firstly, an ultrasonic testing platform for lead seal defects of high-voltage cable terminals was built to detect lead seal defects of different diameters represented by holes and slag inclusions. Then, threshold segmentation and corrosion algorithm processing were performed on the collected fan-scan images of lead seal ultrasonic testing, and quantitative analysis of defects was carried out and compared with the traditional -6 dB quantitative results. Finally, the influence of different diameter defects on ultrasonic quantitative results was discussed. The results show that compared with the traditional -6 dB method, the distance error measured by this method is reduced by 5%, the defect size accuracy is improved by 10%, and the accuracy rate is more than 85%. However, when the diameter of the lead seal defect increases, the defect measurement error also increases. This method verifies that the phased array ultrasonic technology can detect and quantify the lead seal defects of high-voltage cables efficiently and intuitively. It provides an important reference value for the engineering application of lead seal defects of high-voltage cables, which is helpful to improve the quality of lead seal process of high-voltage cable terminals and ensure the safety of power grid operation.