The present invention relates to a method and a device for performing PCO reaction. The method comprises steps of: guiding an air flow into a PCO reactor, which PCO reactor contains photocatalyst for the PCO reaction； splitting the air flow into a first stream and a second stream, wherein quantity of the first stream being less than quantity of the second stream； obtaining the humidity of the photocatalyst； controlling the humidity of the first stream according to the obtained humidity of the photocatalyst； adjusting the humidity of the photocatalyst by guiding the first stream to the photocatalyst； guiding the second stream through the photocatalyst； and illuminating the photocatalyst with light. By controlling the humidity of air passing through the photocatalyst oxidation based on the obtained humidity of the photocatalyst, PCO reaction can be performed effectively.
