The invention relates to the sewage treatment technical field, and discloses a filler and a preparation method thereof; the filler is a polyurethane foam filler containing an activated carbon powder and is prepared by foaming and processing 30-45% of polyether, 10-15% of liquid paraffin, 5-10% of triethylamine, 2-5% of a foam stabilizer, 5-10% of dichloro methane, 3-5% of TDI, 3-7% of activated carbon, 1-3% of stannous octoate and the balance water. The filler has the advantages of excellent gas dynamics performance, small volume density, large specific surface area, and high adsorption efficiency, is not easy to lose in the sewage treatment process, and promotes microorganisms to stably breed and grow, so as to ensure that the filler has good biochemical degradation effect.