The invention discloses a preparation method of active carbon. The preparation method comprises the following steps: (1) performing drip washing on residues obtained in the process of producing ethanol through cellulose fermentation so as to wash off organic acids and inorganic salts in the residues; (2) soaking the washed residues in the step (1) by using a mixed solution of chitosan, glutaraldehyde and sodium dodecyl benzenesulfonate; (3) putting the soaked residues in the step (2) into a carbonizing furnace, heating to 150-200 DEG C in an air atmosphere, treating for 20-60min, then raising the temperature to 600-800 DEG C in an inert atmosphere, and carbonizing for 60-240min (4) after carbonizing, inflating water vapor or CO2 for treating for 10-30 min, and cooling to prepare the active carbon. By the preparation method, with biomass-fermented residues obtained during production of the cellulosic ethanol as raw materials, the active carbon with a high adsorptive property is prepared.