The invention belongs to the technical field of batteries, and particularly discloses a preparation method of a nano-micron lithium manganese phosphate/carbon composite positive electrode material. The method comprises the steps of carrying out the thermal treatment on a raw material solution containing a manganese source, a lithium source, a phosphorus source, the hexamethylenetetramine and the ethylene glycol at 70-80 DEG C in advance, then carrying out the thermal treatment on the solvent at 160-200 DEG C, mixing the prepared nano-micron lithium manganese phosphate with a macromolecule carbon source, drying and calcining at 500-650 DEG C in a protective atmosphere. The primary particles of the prepared material are nano-scale, grow preferentially and are distributed uniformly, so that the diffusion of the lithium ions in the material is facilitated. The secondary particles are micron-scale, so that the structure stability is facilitated, and the prepared material has the relativelyhigher charge-discharge capacity, excellent cycling stability and good rate capability.