Abstract: The thermal vacuum test for solar array EM panels is a critical test to evaluate the solar panel's adaptability in the thermal vacuum environment. In the test, the cell circuits are tested for a long time with several cycles. The main testing technique is to collect the cell circuits' output load voltage in real-time and with high-frequency(no less than 10Hz). By comparison of every cycle and everyday's data, the short-circuit and open-circuit faults could be detected in time. With this technique, the abnormal load voltage sags are identified in time and thereafter the cell circuits' welding quality problems could be detected in the solar array EM panels of a spacecraft. Accordingly, the welding of the cell circuit is improved, and the EM panels are reproduced and retested. With the same technique, the retesting results indicate that the EM panels with improved welding can pass the thermal vacuum test. Finally, the improved EM panels' performance on the orbit justifies the validity of this testing technique.