Abstract: In the vacuum thermal test for the SAR satellite, special measures should be taken to absorb the high power microwave emitted by the transmitter-receiver subassembly to protect it from damage. This paper introduces a new heat flux simulation device, with the functions of both absorbing the wave and simulating the heat flux. In order to verify the validity of the microwave absorbing heat sink, a calibration experiment is carried out. The experimental results show that the heat sink can meet the requirements of external flux precision for the thermal vacuum test, with a bias not more than 4%. The heat sink has been successfully applied in the thermal vacuum test of a SAR satellite.