Abstract: In this study, a series of catalysts with different Fe₃O₄ to iron carbide ratios were obtained by carburizing the α-Fe₂O₃ precursor prepared by co-precipitation method, under various carburization conditions. XRD, Mössbauer spectroscopy, XPS, and Raman spectroscopy were used to characterize the bulk and surface phase compositions of the Fe-based catalysts. The results show that increasing the carburization temperature and prolonging the carburization time lead to higher iron carbide concentration. To explore the active phase of CO₂ formation, the catalysts were tested under different reaction conditions by tuning either CO conversion or H₂O partial pressure. It turns out that the catalytic performance of the Fe-based catalyst in the FTS and water-gas shift (WGS) reactions is influenced by both the content of iron carbide and the degree of carbon deposition. Under typical Fischer-Tropsch reaction condition, the CO₂ selectivity is determined by the CO conversion rather than the Fe₃O₄ content in the catalyst, meaning that the WGS reaction is here limited by the kinetic factors. On the contrary, adding H₂O to the reaction gas results in the trend that higher CO₂ selectivity is promoted by higher content of Fe₃O₄ in the Fe-based catalyst. It seems that Fe₃O₄ is the main active phase for the WGS reaction in the iron-based catalyst for FTS. These results provide a new insight into the active phase of CO₂ generation on the Fe-based catalysts, which could be the theoretical basis for the design of new industrial FTS catalysts with low CO₂ selectivity.