Abstract: Quantitative research was conducted on the use of silica gel and its long-term control effect under three representative air exchange rates (AER) of showcases, and pre-corroded iron sheet samples were placed to verify the control effect. The results showed that when the AER was not higher than 0.42 d^-1, the passive control method could create a showcase microenvironment with relative humidity lower than 20% for iron cultural relics in a period of at least three months. The iron sheets with akaganeite (β-FeOOH) in the showcase did not change significantly, but the rust layer on the samples placed outside of the showcase had thickened, and there were visible shell products, cracks and spalling, which further proved the effectiveness of the passive control method.