The invention provides a lithium manganese phosphate/carbon composite cathode material and a preparation method thereof as well as a lithium ion battery. The preparation method provided by the invention comprises the following steps: firstly, carrying out hydrothermal reaction and solid-liquid separation on a manganese-containing reaction mixture to obtain a manganese-containing compound; and secondly, mixing the manganese-containing compound, a phosphorous-containing lithium salt and a carbon source in a solvent to obtain a reaction precursor, and calcining the reaction precursor in inert gasto obtain the lithium manganese phosphate/carbon composite cathode material. The invention also provides the lithium manganese phosphate/carbon composite cathode material prepared according to the method and the lithium ion battery containing the cathode material. The preparation method provided by the invention has the advantages of simple technique, easily-controlled process, low cost and highyield; the morphology of the lithium manganese phosphate is effectively controlled; the lithium manganese phosphate/carbon composite cathode material provided by the invention has diversified morphology and high specific capacity and cyclic stability.