The reflux liquid in the upper column of the distillation column is throttled by the lower column to the liquid air and liquid nitrogen of the upper column, and the reflux ratio of the upper column is regulated by the liquid nitrogen throttle valve. When the liquid air and liquid nitrogen pass through the throttle valve, the pressure will decrease, and the corresponding saturation temperature will also be reduced. If the supercooling degree of liquid is not enough, some liquid will be vaporized. The higher the gasification rate is, the lower the reflux ratio of the upper column is, and the purity of nitrogen is affected. Therefore, in order to improve the supercooling degree of the liquid nitrogen in the night sky through the throttle valve, a subcooler is generally set up to increase the supercooling degree of the liquid nitrogen in the liquid air.