This paper reported Fe isotope compositions for pyrite in the Mn-bearing series of Gucheng manganese ore deposits in Hubei province, we try to use Fe isotope to constraint processes for pyrite formation and sedimental environment during the early stage of Cyrogenian Datangpo Formation. Our results show δ56Fe values ranged from -0.13‰ to +0.54‰ with a mean of 0.22‰, indicating those pyrites are enrichment in heavier Fe isotope relative to ambient seawater (δ56Fe=0~-0.5‰). We supposed partial oxidation of the initial ferrous iron to ferric iron and the following near-quantitative transformation of iron-oxide or iron-hydroxide into pyrite during diagenesis is responsible for the positive Fe isotopes values for those pyrites in the Mn-bearing series. Thus, we assumed that the deep ocean is gradually oxidized after Tiesi’ao deglaciation, but is not totally oxidized.