Abstract: Under magnetic field conditions, common electromagnetic tribological systems include electromagnetic braking and electromagnetic ejection. Magnetic field promotes the formation of oxidation film on the surface of iron-based friction materials, which changes the friction mechanism and wear mechanism of iron-based materials and improves their wear resistance significantly. Considering practical service conditions of friction couples, the formation of oxide film is closely related to the relative humidity. It is necessary to study the effect of relative humidity on the surface oxidation behavior and friction and wear properties of iron-based friction materials under magnetic field. Being of excellent combination of mechanical and magnetic properties, 45 steel was selected as the research friction material. It was studied that the influence of humidity on the friction and wear properties of 45 steel self-matching pairs under magnetic field. Tribology data were obtained by self-made HY-100 pin-disk friction and wear testing machine. The tests adopt R121 standard of the international Organization of Legal Metrology (OIML) and used the saturated salt solution which had the characteristic of stable vapor pressure to control the ambient humidity of the sealed protection box. Under conventional braking mode, the brake pad of maglev vehicle provided braking friction at low speed. Compared with its service conditions, the magnetic field intensity was set to 17.4 kA/m, the contact pressure and the friction speed were kept constant at 1 MPa and 1 m/s respectively. The transformation of friction mechanism and wear mechanism was analyzed through the evolution of friction surface morphology. The degree and state of iron oxidation on the friction surface were characterized by EDS and XPS. The results showed as follow: with the increase of humidity, the friction coefficient of 45 steel self-coupling increased monotonically under magnetic field, however, the wear rate of 45 steel pin first raised to the maximum at 30% RH and then went down until the humidity tended to saturation. When the humidity was at 50% RH, the roughness of surface reached the highest. The surface roughness evolution indicates that the friction mechanism changes from three-body friction to adhesive-fracture mechanism, and the wear mechanism gradually changed from abrasive wear to oxidation wear. With the increase of humidity from 10% to 70% RH, the oxidation content on the friction surface elevated and the oxygen-iron ratio increased from 0.46 to 0.70. However, the effect of increasing humidity on the oxidation rate of 45 steel surface was gradually decreased, and when the air humidity was saturated (>70% RH) was negative. At 70% RH, the iron oxide hydrate was precipitated with the proportion of OH^- decreasing from 77% to 44% and the oxidation state converting from OH^- to FeO_x·H₂O. The Fe⁰ peak gradually decreased until it finally disappears with the increase of humidity, and the surface of the worn pin sample was gradually covered by iron oxide. The proportion of Fe²⁺ on friction surface increased from 49% (10% RH) to 85% (70% RH). The experimental results provided a reference path to control frictional oxidation behavior and improved the frictional properties and environmental stability by the magnetic field.