It has been found that siderite (FeCO₃) could directly hydrolyze microcystin -LR (MC-LR) under anaerobic conditions in previous studies. However, oxygen (O₂) could not be completely avoided during application process. Therefore, it is urgent to study the effect of dissolved oxygen content (DOC) on the degradation efficiency of MC-LR by siderite. The results showed that the degradation rate of MC-LR under O₂ conditions (DOC=1.62~21.87 mg/L) is higher than that under anaerobic conditions (k₀=0.030 16 mg·L^-1·h^-1), and its degradation rate increased with the increase of O₂ content. When the volume of O₂ added was 1.0 mL (DOC=21.87 mg/L), siderite showed the highest degradation rate of MC-LR (0.083 34 mg·L^-1·h^-1), which was 2.76 times higher than that without O₂. According to humic acid (HA) additions and radical capture experiments, it indicated that siderite could directly activate O₂ to produce superoxide radical (·O₂^-), facilitating the oxidation of MC-LR. However, the addition of HA inhibits the selective hydrolysis of MC-LR by FeCO₃. Finally, the LC-ESI/MS analysis of MC-LR degradation intermediates showed that under aerobic conditions, the hydrolysis sites of MC-LR occurred in Mdha-D-Ala and Adda-L-Arg peptides. Bond, under the oxidation of (·O₂^-) produced by the activation of O₂ by Fe^Ⅱ on the FeCO₃ surface, the hydrolyzed product continues to undergo hydroxylation and decarboxylation reactions, and is finally converted into small molecular aldehydes.