Abstract: Oxygen is a kind of indispensable component in graphite-electrode spark-gap switch, and it is used to oxidize the graphite vapor formed by the graphite electrode under the impact of high-temperature arcs to prevent the graphite vapor from condensing into solid powder after the arc is extinguished, thus to avoid damage to the switch. To increase the oxidation ratio of the graphite vapor, the influence of background gas composition and oxygen concentration on graphite oxidation reaction is studied in this paper. The effect of dilution gases N₂, Ar, and He on characteristics of the oxidation reaction are studied. Meanwhile, on the basis of the traditional air-like gas (80%N₂＋20%O₂), the carbon oxidation ratio in the case with 40% and 60% oxygen concentration are studied. According to the thermodynamic parameters and transport coefficients of different gases, the arc temperature characteristics are obtained through the magnetohydrodynamic calculation. The thermal energy intensity at the interface between the arc and the electrode is used as the basis for evaluating the mass loss rate of the graphite electrode. Experimental results show that as the oxygen concentration increases, the oxidation ratio of graphite vapor gradually increases. However when the oxygen concentration is higher than 40%, there is a risk of combustion of the graphite electrode. When the oxygen concentration is kept at 20%, the mass-loss rate of the electrode is smaller when Ar is used as the dilution gas, and the carbon vapor is oxidized more fully in the arc. This indicates that compared with the traditional insulation gas, replacing the dilution gas with Ar or increasing the oxygen concentration to around 40% can both improve the carbon-oxygen reaction efficiency and reduce the residual carbon impurities of the graphite-electrode spark-gap switch.