The new-exposed Xiamaling Formation near Tielingzi village in Jixian are rich of siderites. Recognition of the siderite formation mechanism is crucially important for judging the redox state and geochemical nature of the ancient ocean during Xiamaling Period , even the whole Mesoproterozoic. In this paper, we use carbon isotope to constrain the genesis of these siderites, and then back analyse the paleoceanic environment during this time. The carbon isotope compositions of these siderites range from-19.2‰ to -7.8‰,with an average of ~-15.0‰，which deviate significantly from the δ13C values of normal marine carbonate rocks, but close to carbonate or bicarbonate originated from organic matter. This illustrates that the carbon in these siderites originated from organic matter, which is also to say, these siderites were transformed from ferric oxides or hydroxides during the early stage of diagenesis, rather than precipitated directly out of seawater. The iron in the ocean firstly precipitated as iron oxides or hydroxides, and buried with organic matter in the seafloor hypoxia zone. Then during the early stage of diagenesis, organic matter was oxidized to CO2 by Fe3+,while Fe3+ was reduced to Fe2+ by organic matter，and siderites were formed by the combination of these resultants. Accordingly，thermally-induced decarboxylation of organic matter offered a portion of CO2 to make more Fe2+ exist in strata as siderites. So we can infer that in Yanliao basin, at least, the ancient ocean during Xiamaling Period had been wildly oxidic, and the degree of oxidation can make iron oxidized to ferric iron. Also the strata are low in sulfur, indicating that the ancient ocean is sulfur -poor . However，during Xiamaling Period, this wildly oxidic and sulfur-poor paleoceanic feature in Yanliao basin is different from the traditional stratified paleoceanic model, so we need more in-depth and systematic research into this issue.