The invention provides a method for preparing a positive electrode material of a lithium ion battery. The method comprises the steps of uniformly mixing a lithium source and an acidic chelating agentin deionized water according to a molar ratio for a reaction to obtain a mixed solution A, wherein the molar ratio of Li to the chelating agent is 1: (0.1-0.5); uniformly mixing an iron source, a phosphorus source and the acidic chelating agent in deionized water according to a molar ratio for a reaction to obtain a mixed solution B, wherein the molar ratio of Fe to P to the chelating agent is 1:1:(0.25-0.5); uniformly mixing the mixed solution A and the mixed solution B according to a molar ratio for a reaction to obtain a mixed solution C, wherein the molar ratio of Li to Fe is 1: (0.98-1);conducting spray drying on the mixed solution C, and conducting sintering in a non-oxidizing atmosphere to obtain the LFP/C positive electrode material, wherein the sintering time is 8-12 hours, and the sintering temperature is 600-800 DEG C. According to the method, semi-solid phase preparation of lithium iron phosphate serving as the positive electrode material of the lithium ion battery is achieved. Compared with a conventional solid phase method, the method has the advantages that a time-consuming and energy-consuming grinding process is not needed, the synthesis time can be effectively shortened, and the production efficiency is improved.