Composite carbon source is an efficient, rapid, low-consumption and non-toxic small-molecule carbon source supplement. Its main function is to remove total nitrogen, and it combines the advantages of several external carbon source agents. It has stable chemical properties, a fast denitrification rate, low sludge production, quick adaptation of sludge strains, and good nitrogen removal effect. The treatment cost is lower than that of several other conventional carbon source agents. It is suitable for the dosing treatment in various sewage plants, meeting the water quality discharge requirements while achieving the maximum economic effect. It is a stable low-cost carbon source supplement.