Abstract: In situ XRD reaction device combined with the online gas chromatography was used to study the oxidation behavior of the effect of H₂O content (4.36%, 1.68%, 0.56%) on the phase and Fischer-Tropsch synthesis (FTS) performance of the single phase Fe₅C₂. The results show that the oxidation rate of the Fe₅C₂ phase increases with the increase of the content of injected H₂O. Meanwhile, the particle size of Fe₅C₂ phase decreases and more active sites exposes during the H₂O oxidation, resulting in the increase of the FTS activity. Furthermore, the FTS activity increases with the increase of the oxidation times, but the selectivity of CH₄ increases and the C₅₊ selectivity decreases gradually.