Abstract: Due to the excellent nuclear property, high thermal conductivity, high boiling point and chemical inertness of lead-bismuth (Pb-Bi) alloy, the Pb-Bi fast breeder reactor, which uses Pb-Bi as the coolant, is one of the six reactor types for the Generation IV reactor. Meanwhile, the reactor structural materials are tend to be corroded by the Pb-Bi alloy coolant, therefore, the corrosion behavior of the reactor structural materials is one of the research hotspots in this field. In order to reveal the corrosion behavior of T91 steel(9Cr ferritic/martensitic steel, F/M steel) Pb-Bi alloy coolant with low flow rate, the corrosion experiment of T91 under oxygen controlled Pb-Bi alloy coolant environment is carried out by using the Pb-Bi thermal convection test loop. The surface and cross-section morphologies of T91 steel, and the phase, chemical compositions and thickness of the corrosion layer are characterized. The results show that an oxide film is formed on the surface of T91 steel when the oxygen con- tent (mass fraction) in the flowing Pb-Bi alloy coolant is controlled within 1.0×10^-8-5.0×10^-8 at 330-550 ℃, and that the oxide film increases with the increase of experimental temperature and time. The oxide film consists of two layers, with the loose Fe₃O₄ in the outer layer and dense Fe(Fe_xCr_1-x)₂O₄ in the inner layer. The dense Fe(Fe_xCr_1-x)₂O₄layer can inhibit the dissolution corrosion of the T91 steel in the Pb-Bi alloy coolant.