The invention belongs to the field of modification of active carbon and mainly relates to an iron-loaded modified active carbon and a preparation method thereof. The preparation method of the modified active carbon comprises the following steps: mixing powder active carbon with iron salt, dissolving a mixture in distilled water at normal temperature, and dipping via ultrasonic wave; and regulating the pH to be 7-8 via an alkaline solution, standing for 24 hours, filtering via a vacuum pump and drying to obtain the modified active carbon. Through the iron-loaded modified active carbon and the preparation method thereof, the active carbon is prepared from livestock and poultry breeding waste cow dung; a modification method of active carbon is provided on the basis of comprehensive utilization of the agricultural waste; the defect of high time consumption of the conventional direct dipping method is abandoned; due to the adoption of an ultrasonic wave load method, the removal efficiency of phosphorus by the active carbon is improved; significant social and economical benefits can be achieved.