A method for preparing a cathode active material of lithium battery is shown. The method includes providing MnOOH and lithium source material, and mixing the MnOOH and the lithium source material in a liquid solvent to achieve a mixture. Then, the mixture is dried to remove the liquid solvent, thereby achieving a precursor. A temperature of the precursor is elevated from room temperature to a sintering temperature of about 500° C. to about 900° C. at a uniform rate, and the precursor is sintered at the sintering temperature for about 3 hours to about 24 hours.