The relationship between iron precursors and Fischer-Tropsch synthesis (FTS) performance is not well understood due to the lack of evidence for the role of the iron valence state. Here, the iron precursors of FeCl2 center dot 4H(2)O and FeCl3 center dot 6H(2)O containing the same anion were selected to study the single iron valence state-dependent FTS performance. The catalysts were in situ constructed on 5, 20, and 100 ppi Fe foam substrates by a straightforward hydrothermal method. The catalysts prepared by FeCl2 center dot 4H(2)O exhibit higher CO conversion, lower methane selectivity, and higher C-2-C-4 olefin-to-paraffin ratio, which can be mainly attributed to the higher Fe2+ content (Fe3O4) in the unreduced state, easier reduction, uniform particle- and pore-size distributions, and a lower degree of agglomeration during the FTS process. Our current findings reveal that iron-based catalysts could be modified simply and directly by changing the percentage of Fe(II)/Fe(III) in the raw materials.