The invention discloses a preparation method of a Fe (III)-containing magnetic nano material, relates to a preparation method of an iron-containing magnetic nano material, and aims at solving the problems that in an existing preparation method of an iron-containing magnetic nano material for adsorbing arsenic, the reaction temperature is high, energy consumption is high, and the reaction conditions are complex. The preparation method comprises the steps that 1, magnetic particles are prepared; 2, blending is performed to obtain the Fe (III)-containing magnetic nano material. The invention further discloses application of the Fe (III)-containing magnetic nano material, and the Fe (III)-containing magnetic nano material is used for adsorbing arsenous acid ions by serving as an adsorbing agent. The preparation method has the advantages that energy consumption is low, and the reaction can be performed under stirring at room temperature; environmental friendliness is achieved, cheap metal salts FeCl3.6H2O and FeCl2.4H2O and sodium carboxymethylcellulose are used as reagents, deionized water is taken as solvent, and no template or other toxic reagent needs to be added; the yield is high, and large-scale preparation can be achieved. The preparation method is mainly used for preparing the Fe (III)-containing magnetic nano material.