The invention discloses a method for preparing a biochar/ammonium vanadate lithium ion cell cathode material. The method comprises the steps of cleaning and cutting a biomass preformed body into pieces, drying the biomass preformed body, and adding concentrated nitric acid for homogeneous reaction, so that a reaction product is obtained; conducting suction filtration and drying on the reaction product, and carbonizing the reaction product in a tubular atmosphere oven, so that a carbonized product is obtained; conducting washing and suction filtration on the carbonized product, and drying the carbonized product, so that porous biochar is obtained; finally, adding biochar into a NH4VO3 solution obtained from deionized water by means of ammonium metavanadate, and placing the mixture in a hydrothermal induction heater, so that the biochar/ammonium vanadate lithium ion cell cathode material is obtained. The obtained biochar/ammonium vanadate composite material has excellent conductivity, and specific capacity and stability are also improved remarkably. The preparing method is simple, reaction temperature is low, reaction time is short, subsequent processing is not needed, environmental friendliness is realized, and large-scale production can be achieved with the method.