The invention relates to a preparation method of positive electrode material of an iron-doped lithium manganate acid lithium ion battery. The method comprises the steps of (1) feeding a chelating agent, an iron source, a manganese source and a lithium source into water, and evenly stirring to obtain mixed solution, wherein the molar ratio of the chelating agent to the iron to the manganese to the lithium is (2-4):x:(2-x):(1.02-1.08), and x is more than 0 and less than or equal to 0.2; (2) drying the mixed solution by distillation to obtain a precursor; (3) carrying out predecomposition on the precursor at the temperature of 150-250 DEG C to obtain a pre-decomposed product; (4) grinding the pre-decomposed product to obtain a ground product; (5) carrying out heat treatment on the ground product at 650-800 DEG C, and cooling to obtain the positive electrode material of the iron-doped lithium manganate acid lithium ion battery. The method has the advantages of being simple in technology; the prepared positive electrode material is high in purity, small in particle size, high in specific capacity and rate capability and long in cycle life, and the demand of the lithium ion battery for practical application can be met.