By utilizing the physical characteristics of ammonia and air condensation at different temperatures, the air which cannot be liquefied in the refrigeration system is separated from the air separator.When air separator is operated, the cold source used enters the coil of air separator by throttling ammonia liquid from the reservoir and evaporates the heat of mixture gas in the coil. Ammonia gas in the mixture gas is liquefied after being cooled. Because air can not be condensed and separated, it is discharged through the vent valve, then the liquefied ammonia enters the refrigeration system and is reused.