Provided are a cathode active material including polycrystalline lithium manganese oxide and a boron-containing coating layer on a surface of the polycrystalline lithium manganese oxide, and a method preparing the same. Since the cathode active material according to an embodiment of the present invention may prevent direct contact between the polycrystalline lithium manganese oxide and an electrolyte solution by including the boron-containing coating layer on the surface of the polycrystalline lithium manganese oxide, the cathode active material may prevent side reactions between the cathode active material and the electrolyte solution. In addition, since limitations, such as the Jahn-Teller distortion and the dissolution of Mn2+, may be addressed by structurally stabilizing the polycrystalline lithium manganese oxide, tap density, life characteristics, and charge and discharge capacity characteristics of a secondary battery may be improved.