Disclosed is a method of preparing a cathode electrode material for a secondary battery, including a hydrate precursor preparation step of preparing a manganese phosphate hydrate precursor using a coprecipitation process, a synthetic powder preparation step of preparing a synthetic powder by mixing the manganese phosphate hydrate precursor in a powder form with lithium phosphate and carbon, an oxide material powder preparation step of preparing a lithium manganese phosphate oxide material powder by milling and annealing the synthetic powder, a composite powder preparation step of preparing a composite powder by mixing the lithium manganese phosphate oxide material powder with a Li2MnO3-based cathode material, and a slurry preparation step of preparing a slurry by mixing the composite powder with a conductor and a binder.