Disclosed is a pretreatment method for improving antioxidation of steel T91/P91 in high temperature water vapor, which includes applying a slurry, containing rare earth oxide, on a surface of a substrate; holding the temperature in a gas mixture environment of inert gas and water vapor after drying; and cleaning away the solid powder left by the slurry on the surface, thereby obtaining the substrate with a surface rich in chromium and having a small amount of rare earth oxide. As a result of the method, the antioxidation capability of steel T91/P91 in the 500-750° C. water vapor environment can be improved, and films rich in chromium oxide can be formed on the surface of steel T91/P91.