The lead seal of cable terminals plays a critical role in the sealing and waterproofing of high-voltage cables, and its reliability is directly related to the safe operation of the cable system. During the manufacturing and installation processes, defects such as cold soldering, loosening, and cracking may occur in cable lead seals, making traditional detection methods insufficient for identifying these internal defects effectively, thus increasing the risk of cable failures. This study systematically investigates the detection methods and effectiveness of ultrasonic technology for lead seal defects at cable terminals. The experiment employs the HS PA20 ultrasonic testing equipment developed by Zhongke and utilizes the design of a flexible probe and a coupling medium water bag to enhance the precision and reliability of ultrasonic detection. Experimental results show that ultrasonic detection can effectively identify inter-layer and internal defects of lead seals, providing scientific evidence and technical support for the on-site non-destructive detection of lead seal defects at cable terminals.