Environmental humidity influences the safety and replacement frequency of Fe-based friction materials such as brake pad and wheel-rail, which are influenced by the formation of oxide films. However, the oxidation mechanisms induced in iron and steel materials as a function of relative humidity (RH) have not been clearly elucidated thus far. Therefore, in this study, normalised 45# steel was tested under braking conditions, with the friction interface positioned perpendicular to the ground. To investigate the effects of RH on the friction and wear performance as well as the oxidative wear mechanism in 45# steel, pin-on-disc friction tests were conducted via a salt solution method at different RH conditions. The results showed that as RH increased, the friction coefficient of 45# steel decreased, whereas the wear rate increased; ultimately, optimal braking performance was achieved at 30% RH. Contrastingly, at 50% RH, the friction coefficient and wear rate exhibited abrupt changes, the O:Fe ratio on the worn surface peaked, and the wear mechanism shifted from furrow and adhesive wear to oxidative wear. Finally, X-ray photoelectron spectroscopy analyses revealed that with increasing RH, the oxidation intermediates followed a transformation sequence of Fe(OH) 2 → FeOOH → Fe(OH) 3. Ultimately, this study provides essential data and theoretical insights into oxidation processes, supporting the design and performance evaluation of iron-based friction materials.