The body-mounted fluid tube radiator (BMFTR) is a highly efficient heat rejection device for spacecraft. However, the heat rejection rate of the BMFTR is negatively impacted by the presence of solar panels mounted on the exterior of the spacecraft. In this study, a heat transfer model for the BMFTR was developed, and a simulation method was created to investigate the effect of solar panels on the radiator’s performance. The accuracy of both the heat transfer model and simulation method was verified using on-orbit data from the China Tianhe module. It was found that external heat is absorbed by the solar panels, which in turn reduces the performance of the radiator. Furthermore, the decrease in the heat rejection rate was quantitatively evaluated, and it was found to be closely related to the spacecraft’s attitude and the view factor between the solar panel and the radiator. The findings of this study on the impact of solar panels on the BMFTR’s performance are meaningful for future research on spacecraft radiators and on-orbit operations.