Abstract: A lithium-rich manganese-based positive electrode material represented by formula (I). In an X-ray diffraction pattern of the lithium-rich manganese-based positive electrode material, a ratio of a diffraction peak intensity of a Bragg angle near 18.7° to a diffraction peak intensity of a Bragg angle near 44.6° is 1.10 to 1.24. A method for preparing the lithium-rich manganese-based positive electrode material comprises: mixing a lithium-rich manganese-based compound with a lithium remover; and under the effect of the lithium remover, removing part of Li2O from Li2MnO3 in the lithium-rich manganese-based compound, so as to obtain the lithium-rich manganese-based positive electrode material. Due to the reduction of the irreversible product Li2O, the initial coulombic efficiency is improved; because of existence of lithium vacancy and oxygen vacancy, the lithium-rich manganese-based positive electrode material has good rate performance and cycle performance.