Abstract: The spacecraft external components（such as solar array）surface temperature is needed to be reduced to-170℃during thermal vacuum test.The specimen is hard to be cooled to-170℃ by the liquid nitrogen and thermal shroud system of conventional space environment simulator.A negative pressure subcooling system was developed to further reduce the test temperature.The system consists of subcooler，heating system，pressure control system and piping system，utilizes liquid nitrogen to have a lower saturation temperature at negative pressure to obtain a lower liquid nitrogen temperature than conventional liquid nitrogen systems.Negative pressure subcooling system was designed in this paper，and an engineering prototype was built and tested.The result of thermal tests shows that the outlet temperature of subcooler can reach lower than-200℃and the period of thermal tests with the lowest temperature of-170℃can be minimized to 40%.