Abstract: In view of two insulation defects of 66 kV plug-in cable terminals, test results of comprehensive AC withstand voltage test, partial discharge detection, cable terminal disassembly, glass transition temperature test, and X-ray imaging detection were analyzed. Results showed that improper installation of stress cone at A-phase cable terminal led to the gap between stress cone and epoxy sleeve, as well as discharge fault in AC withstand voltage test. Impurities in epoxy sleeve of C-phase cable termination were caused distorted of electric field and abnormal partial discharge. In addition, some relevant solutions were proposed, defects could successfully eliminated by remaking and installing cable terminals. Carring out simultaneously AC withstand voltage test with partial discharge measurement and using joint detection method combining a variety of partial discharge measurement methods could effectively improve the accuracy of cable terminal defect diagnosis, which could be of great significance for ensuring the safe operation of the cable line.