Abstract: The plume effect experimental system(PES) is used for the plume effect test of rocket engines and the vacuum heat tests of satellites, with special requirements for the liquid nitrogen system. A new negative system is designed for the liquid nitrogen temperature as low as 70 K. In this system, the liquid nitrogen at a negative pressure is used to have a lower saturation temperature than the atmospheric pressure nitrogen, and to obtain a lower liquid nitrogen temperature than the conventional liquid nitrogen subsystems. The negative pressure liquid nitrogen system consists of the liquid nitrogen delivery subsystem, the atmospheric subcooler subsystem, the negative pressure pumping subsystem, the liquid nitrogen pump subsystem, the heat sink subsystem and the discharge subsystem, where the atmospheric subcooler subsystem and the negative pressure pumping subsystem are combined to serve as the negative pressure subcooler. With the atmospheric subcooler, the negative pressure subcooler can adjust the temperature of the feeding fluid and provide the heat sink with 70-77 K liquid nitrogen for refrigeration, to meet the demand of the engine plume experiment and the thermal vacuum test of satellites.