The invention discloses a method for preparing an NH4V3O8 anode material for a lithium ion battery. The method comprises the steps that ammonium metavanadate is dissolved in deionized water, an NH4VO3 solution is prepared, pre-oxidization short carbon fibers of 1 mm to 3 mm are added, the NH4VO3 solution is transferred into a reaction still after the pH value is adjusted, forced mixing is carried out, after the reaction still is sealed, the reaction still is placed in a water and heat induction heater, and suspension liquid is obtained after a heating reaction; centrifugal separation is carried out on the suspension liquid to obtain a powder product, the powder product is soaked in the deionized water and the absolute ethyl alcohol, repeated washing is carried out, and then drying and grinding are carried out. The NH4V3O8 microcrystal prepared through the method is uniform in chemical component, high in purity and uniform in form, and the electrochemical performance of the material can be effectively improved. The preparation method is simple, low in reaction temperature, short in reaction period, high in productivity, free of any subsequent treatment, environmentally friendly and capable of being suitable for large-scale production.