The invention belongs to the technical field of preparation and application of nano 
composite material, and in particular relates to a method for preparing a magnetic nano metallic iron particle / carbon 
nano tube functional composite material. The invention discloses a method for filling a carbon 
nano tube with magnetic nano metallic iron particles. The method comprises the following steps: carrying out refluxing treatment on the carbon 
nano tube in concentrated 
nitric acid, introducing functional groups such as carbonyl, hydroxyl, carboxyl and the like on the surface of the carbon nano tube, carrying out 
ultrasonic dispersion and electromagnetic mixing on the treated carbon nano tube in 
aqueous solution of bivalent 
ferric salt, filling the bivalent 
ferric salt in the carbon nano tube through 
capillary action, thermally treating the carbon nano tube at 
inert atmosphere, decomposing the bivalent 
ferric salt filled inside the carbon nano tube into ferric 
oxide, and reducing the ferric 
oxide into the magnetic nano metallic iron particles by 
hydrogen to obtain the 
composite material for filling the magnetic nano metallic iron particles into the carbon nano tube, wherein the magnetic nano metallic iron particles can be filled in the carbon nano tubes the calibers of which are 10 to 100 nanometers and which have different structures, and the carbon nano tubes with different calibers as templates can accurately control the size of the magnetic nano metallic iron particles. The method has the advantages of simple process and good 
controllability, can make the magnetic nano metallic iron particles evenly filled in the carbon nano tube with high 
filling rate, and is convenient for industrialized production; and the prepared nano 
magnetic composite material has good application prospect in the fields of high-density magnetic recording materials, wave-absorbing materials, 
electromagnetic shielding materials, 
nuclear magnetic resonance imaging materials.