Abstract: As a new geochemical index, the Fe/Mn ratio of carbonate rocks can be used to restore the redox state of ocean. Both Fe³⁺ and Mn⁴⁺ are insoluble under the oxidation condition, so the dissolved Fe and Mn contents in oxidized seawater are very low. Fe³⁺ and Mn⁴⁺ can be reduced to soluble Fe²⁺ and Mn²⁺ by bacteria under the reduction condition, and the calculation of redox potential shows that the reduction of Mn⁴⁺ is earlier than that of Fe³⁺, thus the bacteria-induced Mn reduction process occurs in the shallower layer of sediments. The soluble Fe²⁺ and Mn²⁺ diffuse upward into seawater to replace the Ca²⁺ in lattice of carbonate rocks. The contents of Fe²⁺ and Mn²⁺ are controlled by the diffusion of porewater in sediments, while the latter is related to the redox state of water-rock interface. Therefore, it can be predicted that the Fe/Mn ratio in carbonate rocks would gradually increase as seawater becomes less oxygenated. The Fe/Mn ratio of dolomites in the Mesoproterozoic Gaoyuzhuang Formation is analyzed to test the hypothesis. It is found that the Fe/Mn ratio of almost all samples is 20-30, significantly higher than that of carbonate rocks in Late Devonian deep water and shallow water platform, which may indicate the low atmospheric oxygen concentration and wide oxygen depletion in ocean in Mesoproterozoic, and the dolomites were formed in the anoxic sediment porewater.