The invention provides a pressure control method and device applied to an air separation system. The air separation system comprises a first pure nitrogen pipeline, a second pure nitrogen pipeline, athird pure nitrogen pipeline, a first dirty nitrogen pipeline, a second dirty nitrogen pipeline, a third dirty nitrogen pipeline and a fourth dirty nitrogen pipeline; the third pure nitrogen pipelineand the first dirty nitrogen pipeline are connected with a nitrogen water tower; and the second dirty nitrogen pipeline and the third dirty nitrogen pipeline are connected with a molecular sieve. Themethod comprises the following steps: the openness of a first valve on the first pure nitrogen pipeline is controlled within 30-35%, and the openness of a second valve on the second pure nitrogen pipeline is controlled within 30-35%; when the molecular sieve is determined in a heating phase or a cold blowing phase, the first openness of a third valve on the third pure nitrogen pipeline is controlled based on a preset upper tower pressure, and the preset upper tower pressure is 36 KPa; a fourth valve on the first dirty nitrogen pipeline is controlled to close; and a sixth valve on the fourth dirty nitrogen pipeline is controlled to close.