The effect of the precursor composition of fused iron catalyst on the performance of Fischer-Tropsch synthesis was investigated. XRD, BET and CO2 adsorption experiments were carried out to provide better insight into the relationship therein. The results showed that the selectivity of C5+ hydrocarbon products was dependent on the mole ratio of Fe2+/Fe3+, which was represented by a hump-shaped curve. Catalysts with precursors containing Fe3O4 phase favored the magnetite spinal formation during F-T reaction, while Fe1-xO-based catalysts were more likely to favor the formation and growth of the iron carbide crystals.