The invention relates to a hydrothermal synthesis method for lithium ferromanganese phosphate anode material of a lithium ion battery, belonging to the technical field of lithium ion batteries. The method has the following processing steps of: step 1, preparing LiMnxFe1-xPO4 through a hydrothermal synthesis reaction: mixing an aqueous lithium hydroxide solution, an aqueous ferrous sulfate solution and phosphoric acid under a stirring condition, after sealing, heating to 150 DEG C to 180 DEG C within 0.5 to 2 h, reacting for 0.5-4 h under the pressure of 0.48-1.0 Mpa, cooling to less than 80 DEG C, and filtering; step 2, mixing with organic matters and drying: mixing a wet filter cake with a soluble carbon source organic matter, and carrying out spray drying or expansion drying; and step 3, carrying out carbon-coated processing: roasting LiMnxFe1-xPO4 carbon source compound powder at a temperature between 600 DEG C and 750 DEG C for 4-6 h under an insert gas condition, and cooling to less than 150 DEG C to obtain a carbon-coated lithium ferromanganese phosphate anode material of the lithium ion battery. The hydrothermal synthesis method disclosed by the invention has the advantages of simple and controllable technology, convenience for operation, low cost, high crystallization degree of products, uniform dispersion, high specific capacity, high conservation rate of the cycling capacity, and the like.