NOVELTY - Hydrothermal synthesis of composite material, comprises: (i) dissolving divalent ferric salt in deionized water, adding phosphorus salt and ascorbic acid, stirring, dissolving lithium salt in deionized water to form a solution; (ii) taking out mixed solution from agitated reactor after hydrothermal reaction, aging, discharging supernatant liquor, then arranging precipitate in a vacuum drying box for drying in vacuum; and (iii) arranging precursor powder obtained in a small ceramic boat, carrying out calcination in a tube furnace in protective condition of feeding an inert gas continuously.    USE - The composite material is useful for high-performance dynamic battery.    ADVANTAGE - The method: shortens the ion transmission path and improves electron transmission efficiency, as the graphene forms a carbon coating on the surface of the granules of the materials; and is utilized for high-power charging and discharging.    DETAILED DESCRIPTION - Hydrothermal synthesis of composite material, comprises: (i) dissolving divalent ferric salt in deionized water, adding phosphorus salt and ascorbic acid, stirring and mixing evenly, dissolving lithium salt in deionized water to form a solution, then dropping the lithium salt solution into the former mixed solution while stirring continuously, where the molar ratio of lithium:iron:phosphorus is 3:1:1, stirring for 1-30 minutes, adding graphene dispersed liquid dispersed by ethyl alcohol to form a grayish-green mixed solution, transferring the mixed solution into a high-pressure reactor, adding magnetic granules, sealing and reacting for 2-10 hours in dimethyl silicone oil bath at 120-180 degrees C; (ii) taking out the mixed solution from the agitated reactor after hydrothermal reaction, aging for 12 hours, discharging the supernatant liquor, washing the precipitate on the lower layer repeatedly using deionized water until sulfate ion is not detected in the washing liquid, then arranging the precipitate in a vacuum drying box for drying in vacuum at a drying temperature of 35-80 degrees C; and (iii) arranging the precursor powder obtained in a small ceramic boat, carrying out calcination in a tube furnace in the protective condition of feeding an inert gas continuously at 500-800 degrees C for 2-10 hours.