The helium is utilized as the primary coolant in the high temperature gas-cooled reactor (HTGR), which contains traces of impurities such as CO, H2, H2O, and CH4 in the ppm range. Although the impurity content is pretty low, it can react with materials used in the intermediate heat exchanger (IHX) or steam generator (SG) to cause the corrosion phenomena at high temperature. Nickel-based alloys have been selected as the candidate materials for HTGR due to the excellent properties, and, in this study, Inconel 617 was the experimental alloy. The high temperature corrosion experiments of Inconel 617 were carried out in the impure helium with various CO levels. A stepwise heating pattern was adopted, and the decarburization behavior of Inconel 617 was observed at high temperatures. This research revealed the corrosion reactions of the alloy under different conditions and established a model to elucidate the mechanism of oxide layer rupture. In addition, an interesting phenomenon was discovered. When the temperature decreased at cooling stage, CO in the environment would be rapidly consumed, which was explained by the corrosion model. Furthermore, as the CO content increased, the decarburization reaction rate decreased. This study provides a reference for the impurities controlling in the helium coolant of HTGR, suggesting that the CO content should be increased to suppress the oxide scale rupture and decarburization.