The invention relates to a flocculating agent for treating domestic sewage. The flocculating agent comprises polyol phosphate, alkylamine polyoxyethylene ether oxide, etidronic acid, polyamino polyether methylene phosphonate, calcium sorbate, methoxybenzoic acid, hydroxyquinoline, methylene dithiocyanate, potassium peroxydisulfate, nanometer titanium dioxide, wollastonite powder, sodium thiosulfate powder, sodium bicarbonate, nanometer zinc oxide, sodium dodecyl sulfate, sodium dodecylbenzenesulfonate, magnesium stearate, a quaternary ammonium salt, coal tar, N-ethylmorpholine, carbon tetrachloride, pentadecanol, starch phosphate, 2-ethylhexyl acrylate, butylated hydroxyanisole, dimethyl diallyl ammonium chloride, aniline, tryptophan, threonine, polycarbosilane and deionized water. The flocculating agent is high in reaction speed, and free of generation of toxic and harmful gas in a process. A product after the reaction is stable and does not decompose into toxic compounds. The flocculating agent is efficient, nontoxic, safe to human bodies before and after the reaction and high in sewage treating efficiency.