The invention discloses a hydrothermal synthesis method for preparing nano-scale carbon-coated lithium iron phosphate, which belongs to the field of lithium-ion battery anode materials and comprises the following steps of: sequentially adding a phosphorus-source solution, an iron-source solution, a carbon source, a lithium-source solution and a boiling-point elevator into a reaction device in sequence, mixing, then heating to the temperature of 60-180 DEG C for reaction under inert gas, cooling after the reaction, filtering precipitates to obtain a nano-scale lithium iron phosphate precursor, and then sintering the nano-scale lithium iron phosphate precursor at the temperature of 400-600 DEG C under the protection of the mixed gas of the inert gas and hydrogen gas. The combination of lithium, iron, phosphorus and the like on a molecular level is realized by the method, and the grains of a product are quite fine and are uniformly distributed; because the boiling-point elevator is adopted, the reaction temperature and pressure of the reaction kettle are decreased; and the hydrothermal synthesis method has the advantages of simple preparation process, short flow, easiness in operational control, low reaction temperature, short time, low energy consumption and easiness in realizing large-scale industrialized production.