The inventive concept discloses a method for preparing a cathode active material containing a lithium manganese oxide exhibiting a reversible phase transition, and exhibiting electrochemical characteristics of the lithium manganese oxide through the reversible phase transition including (A) synthesizing a sodium manganese oxide using a manganese precursor, and (B) reacting the sodium manganese oxide with a lithium precursor to synthesize the lithium manganese oxide, or including (C) directly synthesizing the lithium manganese oxide.