The invention discloses a preparation method of lithium iron phosphate oxide of cathode material of a lithium ion battery, which comprises the following steps: adding ferrous ion aqueous solution withthe concentration of 0.05-2mol/L and ammonium dihydrogen phosphate solution into a reactor according to the ratio of 1: 1; reacting for 0.2-2h under the conditions that the reaction temperature is 50-90 DEG C and the stirring speed is 400-1200rpm; adding excessive hydrogen peroxide and reacting for 0.2-2h, and ageing, filtering, washing and drying to obtain precursor iron phosphate powder; mixingthe iron phosphate powder, lithium ions in a lithium source and carbon atoms in a carbon source according to the mol ratio of 2: 2: 0.1-0.6 by taking absolute ethanol as a medium, and then putting into a planetary ball mill for grinding to obtain precursor solution of the lithium iron phosphate oxide; and then roasting to obtain the lithium iron phosphate oxide with the particle diameter of 0.5-5Mum. The product prepared by the method has even and fine particle diameter distribution, high reaction activity, simple and convenient operation process, simple device and easy control, and effectively improves the electrochemical property and reduces the cost.