Abstract: The spaceborne multiplexer is the key component in the satellite payload system. Its RF performance in the vacuum environment, such as the insertion loss and the fluctuations, needs to be accurately measured to evaluate the application characteristics of the component. In this paper, the causes of inaccurate and unstable measurements of the multiplexers in the thermal vacuum test are analyzed based on the configuration and the working principle of the test system, and it is shown that the temperature variation of the test cables is the principal factor affecting the measurement accuracy. Five methods are proposed accordingly to improve the measurement accuracy, including the reference cable assistance check, the introduction of the waveguide test system, the coverage of the test cables for the temperature control, the time domain analysis to offset the performance variation of the test cable, and the real-time channel calibration of the test cables. The actual measurement and calibration are provided. After a balancing among the five methods, we recommend the actual measurement and calibration method, for the industry to improve the multiplexers measurement accuracy in the thermal vacuum test.