Abstract: In the space vacuum thermal test, the non-heat-sink configurations bring about the additive heat flux which increases the background flux during the thermal balance test and affect the validation of the system thermal design. Therefore, the requirements of the low temperature maintenance and the temperature rising speed are difficult to achieve, and the environment adaptability of the spacecraft can not be validated adequately. In this paper, the chief factors of the additive heat flux are analyzed and the corresponding countermeasures are suggested. These methods were applied in the tests successfully.