The invention belongs to the field of lithium batteries, and provides a preparation method of a high-capacity iron-based lithium ion battery cathode material, that is, alpha-LiFeO2 to overcome the defects of the complicated synthesis method of the existing alpha-LiFeO2 cathode material, low electrochemical performance, complicated structure, unpurified product and the like. The method for synthesizing at the room temperature is adopted, alpha-LiFeO2 is prepared by strictly controlling the molar ratio of Li<+> / Fe<3+>, various reactants are dissolved into absolute ethyl alcohol and are directly synthesized into alpha-LiFeO2 at the room temperature through magnetic stirring, metal lithium is introduced in the reaction process to serve as a reduction protective agent, an alpha-LiFeO2 product rich in lithium is obtained, the product is centrifuged, separated, washed, dried, grinded, refined and dried to obtain the lithium ion battery cathode material alpha-LiFeO2; the material has no impurity, is high in purity, single in phase and even in particle size distribution, the first specific discharge capacities respectively reach 450 mAh / g, 260 mAh / g and 202 mAh / g at the charge-discharge rates of 0.1 C, 0.2 C and 0.5 C; and in addition, the manufacturing cost is low, the synthesis method is simple, and the preparation method is suitable for large-scale industrial production.