Abstract: Based on the operating conditions of the cooling tubes in the fusion reactor blanket, the effect of dynamic diffusible hydrogen on the oxide film of 9Cr-ODS steel thin-walled tubes was investigated after 400 h corrosion in high-temperature high-pressure (325 ℃/15.5 MPa) deoxygenated water. The results indicate that a dual-layer structure was exhibited by the oxide film on 9Cr-ODS steel, with the inner layer composed of (Fe, Cr)₂O₄ and Cr₂O₃ and the outer layer consisting of Fe₃O₄. The stability of the oxide film was reduced by diffusible hydrogen, with defects and cracks generated on its surface, while no significant interfacial penetration was observed.