The invention relates to a sludge-based activated carbon and a preparation method thereof. In the invention, according to composition of sludge, a certain amount of biomass material (such as corn straw, wheat straw, peanut shell and the like) is appropriately added, mixing to be uniform is carried out, and carbon content of agricultural waste such as crop straw, peanut shell and the like is relatively higher, thus adsorption capacity of the activated carbon is improved (can be improved) when the crop straw and the peanut shell are added into sludge. Activating agent with certain concentration (such as ZnCl2, KOH, KCO3 and the like) is added into the sludge which is mixed to be uniform, impregnating and activating treatment is carried out, and then high temperature activation is carried out, thus the sludge-based activated carbon can be obtained. The sludge-based activated carbon provided by the invention has the advantages of simple process, low cost, and high specific surface area; meanwhile, a way is provided for recycling sludge and treating the agricultural waste, and hazard-free treatment, reduction and reutilization on waste treatment are realized.