The invention discloses a preparation method of a filter core capable of fast and efficiently adsorbing caesium. The preparation method comprises the following steps of based on the weight of a substrate material, adding 1-3wt% of a coupling agent and 1-10wt% of ammonium phosphomolybdate into the substrate material, heating them to a temperature of 160-220 DEG C, uniformly stirring so that the substrate material is coupled with ammonium phosphomolybdate, carrying out filament spraying by micropores of 5 microns, wherein the sprayed filaments form an adsorption filter core having the thickness of 5-50mm on a revolving cylindrical framework material, uniformly coating ammonium phosphomolybdate on an outer surface of the adsorption filter core, and carrying out heating stable-aging at a temperature of 80-120 DEG C for 3-12h. The filter core has good water permeability and oxidation resistance, can be immersed in hot water at a temperature of below 100 DEG C for a long time, can resist irradiation decomposition, has stable adsorption performances, a fast detection rate, high efficiency and high precision, and can determine an adsorption rate without stable cesium measurement.