The corrosion of heat transfer tubes in high-temperature gas-cooled reactor steam generators is a continuously monitored issue. Building on previous research, this study investigates the oxidation kinetics of a typical candidate alloy, Incoloy 800H, during trace air ingress accidents. Corrosion experiments were conducted under two different He-based atmospheres with ppm levels of O2 (varying oxygen concentrations and durations). The impurity gas content during the experiments was monitored in real time using a gas chromatograph, which yielded the oxidation behavior of the alloy. The oxidation rate indices np and nt), parabolic rate constants (kp and kt), and the constant L for evaluating internal oxidation rates were calculated. The results indicate that discontinuous oxidation experiments conducted under low oxygen concentrations may fail to capture the key oxidation kinetics of the alloy. The np, nt, kp, and kt values of Incoloy 800H obtained from the two sets of atmospheric conditions are close to each other and consistent with kinetic calculations in air. The differences in the length of the linear oxidation region and the oxygen concentrations are the main reasons for variations in the alloy’s oxidation behavior.In the He-ppm O2 atmosphere, a gas chromatograph can continuously and accurately monitor the oxidation behavior of the alloy, thereby replacing the thermogravimetric analyzer for continuous measurement and enabling accurate calculation of kp. This research provides insights for studying oxidation and corrosion in heat transfer tubes.