The invention discloses a synthesis method of battery positive material lithium manganese phosphate. The synthesis method comprises the following steps: adding electrolytic manganese into an acetic acid solution, performing a sealed reaction to form manganese acetate, and discharging hydrogen; adding battery-grade lithium carbonate into phosphoric acid to form lithium dihydrogen phosphate; addingthe prepared lithium dihydrogen phosphate into manganese acetate to synthesize lithium manganese phosphate; circularly applying the evaporated acetic acid to preparation of manganese acetate, and grinding and packaging lithium manganese phosphate. The synthesis method disclosed by the invention has the advantages of simple synthesis technology of lithium manganese phosphate, accurate measurement,no three wastes, low cost, prevention of lots of wastewater hard to deal with and environmental friendliness.