Abstract: For the simulation of the ultra-low heat flow in the thermal balance test of spacecraft, a large-area heat flowdynamic simulation system is developed to replace of the traditional fixed infrared heating cage, which is supposed to run continuously under high and low temperatures for a long time. The system can produce a high heat flow or an ultra-low heat flow on the spacecraft surface by adjusting the relative position of infrared heating cage with respect to the spacecraft surface without opening the vacuum tank. The minimum switch time between the two working conditions is no more than 3 min, and the minimum simulated heat flow is not more than 20 W/m² by using the system. The system was successfully applied in the thermal balance test of a spacecraft. It is shown that the system can effectively reduce the external heat flow received by the surface of the spacecraft in the low temperature condition, and reduce the surface temperature of the spacecraft in the low temperature condition, and the stand-alone temperature on the surface by 3.5~10 ℃.