Abstract: The helium coolant in the primary circuit of the high-temperature gas-cooled reactor (HTGR) contains trace of impurities such as CO, H₂, H₂O, and CH₄, which have an adverse effect on the structural materials at elevated temperature. Mainly, the corrosion behaviors include oxidation, decarburization, and carburization, depending on the impurity composition and corrosion temperature. Inconel 617 is the reference candidate material for steam generators of HTGR, which may be corroded by trace impurities in helium at high temperature. In order to explore the corrosion mechanism of the superalloy in impure helium and establish a prediction model of decarbonization phenomenon, the corrosion experiments of Inconel 617 were carried out at 980℃ in the impure helium. The gas phase data and corrosion behaviors of the alloy were analyzed by gas chromatograph (GC), field emission scanning electron microscopy (FESEM) with energy-dispersive X-ray spectroscopy (EDS) system, and X-ray diffraction (XRD). The mechanism of decarbonization is elucidated by chemical thermodynamics, which indicates that the driving force of carbon transfer is the carbon potential difference between alloy and environment, and more specifically, the carbon activity difference. Then, the prediction model of the decarbonization reaction was established. The critical temperature (T_A) at which the corrosion behavior occurs can be obtained by thermodynamic calculation, and it is a function of the partial pressure of carbon monoxide. This model is in good agreement with the experimental data in this study and previous works with different contents of carbon monoxide. The results show that even if the impurity level is very low, it can still induce corrosion behavior. On this basis, the effects of pre-oxidation and corrosion temperature on the decarbonization reaction of alloy were investigated. When the temperature is reduced, there is no more obvious decarbonization phenomenon of the alloy, which indicates that it is an effective way to avoid decarbonization. However, after the pre-oxidation in the air at high temperature, Inconel 617 still has carbon loss, which may be due to the imperfect oxide layer formed in the air. In order to improve the compactness of the alloy oxide layer, surface modification such as coating may be more effective. For the impurity content, this study shows that Inconel 617 has strong decarburization behavior in the impure helium with very low impurity content. Therefore, the low level of impurity is not the goal of coolant purification in HTGR, and the more reasonable impurity scheme should be selected according to model prediction and experimental analysis.