Abstract: The requirement of the large solar simulator in a thermal balance test is being enhanced because of the complex structure and the surface thermal-optical properties of the spacecraft. On the basis of the off-axle solar simulator structure, the origin and the effect of the additional heat flux in the vacuum chamber are analyzed when the solar simulator is used in the thermal balance test. According to the simulation calculation of the additional heat flux with KFTA and KM6 simulators, some improvement suggestions in using the solar simulator in the thermal balance test are made.