Asteroid and comet exploration spacecrafts are subjected to various forms of external space heat flux during different flight phases, necessitating the use of infrared cages and solar simulators for accurate space heat flux simulation under distinct thermal balance test conditions. In thermal balance tests utilizing solar simulators, non-operational resident infrared cages (used for other test conditions) positioned parallel to the simulator’s beam introduce additional background heat flux through optical occlusion, even though they do not directly heat the spacecraft. This unintended thermal interference compromises the accuracy of the spacecraft’s external heat flux simulation. This study establishes a comprehensive simulation model that incorporates resident infrared cages, solar beams, vacuum chambers, L-shaped test bracket and spacecraft components to quantify the magnitude of background heat flux under three solar simulator-based test conditions. The maximum impact of infrared cage interference on the heat flux simulation accuracy of the solar simulator is assessed, leading to the development of the targeted technique for suppressing background heat flux. This optimization significantly enhances the fidelity and effectiveness of the thermal balance test.