The invention provides modified biochar, a preparation method and application thereof. According to the modified biochar, biomass is used as a raw material, and is subjected to carbonization pyrolysisthrough mixing modification with potassium permanganate and ferric chloride to prepare iron-manganese oxide loaded biochar, and the biochar is modified with chitosan to obtain the modified biochar, wherein the raw material is one of straw, rice straw, rice husk, peanut shell, corncob, camellia oleifera shell and cottonseed hull. According to the invention, the prepared modified biochar is large in specific surface area, developed in pore and multiple in surface oxygen-containing functional groups; the modified biochar is applied to treatment of heavy metal organic matter contaminated soil, onthe basis of promoting adsorption of heavy metal and organic matter in the contaminated soil, a good solidification and stabilization effect can be achieved on the heavy metal organic matter; and themodified biochar material can stably exist in the soil for a long time, and has long-term restoration.