The invention provides a composite material for water treatment, which is granular carbon loaded with hydrous ferric oxide and nano iron particles. The preparation method comprises the steps of loading the hydrous ferric oxide on active carbon; loading nano iron on the active carbon; and drying the active carbon loaded with hydrous ferric oxide and nano iron particles for spare. Further, the granular carbon can be loaded with CTAB. The material has the application that the composite material is added into water containing perchloric acid ions, and a primary cell is formed inside a water solution with ion concentration by adopting the adsorption and complexing actions and the loaded nano iron to reduce and degrade the perchloric acid ions. The method can adsorb, degrade and remove the perchloric acid ions containing in the tap water leaving factory and perchloric acid ions in the de-ionized water so as to standardize the quality of the drinking water.