The invention relates to sludge-based biochar, in particular to a method for preparing carapace-doped sludge-based biochar. The method includes air-drying municipal sludge, drying the air-dried sludge in a drying oven at 105 DEG C to reach a constant weight, and crushing the dried sludge to reach the particle size of 4-8 mm; degreasing carapaces with absolute ethyl alcohol for 30 minutes at 60 DEG C, drying degreased portions in the drying oven at 105 DEG C, and crushing the dried degreased portions to reach the particle size of 5-8 mm; mixing the crushed municipal sludge with the crushed carapaces sufficiently according to a weight ratio of 1-3:1, grinding and screening the power mixture by a 200-mesh screen; disposing the screened mixture in a muffle roaster, feeding nitrogen and heating so as to obtain a pyrolysis product; subjecting the pyrolysis product to immersion cleaning with 1-3 M hydrochloric acid for 5 minutes, repeating for 5 times, drying a filtered product in the drying oven at 105 DEG C to reach a constant weight so as to obtain the carapace-doped sludge-based biochar. The method saves preparation cost by turning the sludge and the carapaces into wealth and improves the sludge-based biochar quality and the wastewater treatment performance to a great extent.