The invention discloses a preparation method of a flocculating agent for environmental hormone polluted underground water, which comprises the following steps: by taking a dimethyl diallyl ammonium chloride monomer as a raw material, adding a metal ion chelating agent, persulfate and a high-temperature initiator, introducing nitrogen, carrying out heating reaction, then adding a chain transfer agent ethylene glycol, continuing heating reaction, finally adding hydrogel, performing heat preservation reaction, and performing concentration and spray drying. According to the preparation method of the flocculating agent, on the basis of a traditional preparation method of the flocculating agent poly dimethyl diallyl ammonium chloride, the molecular mass of a product is reduced by adding the chain transfer agent ethylene glycol, meanwhile, due to the fact that the molecular mass of the product is reduced, the product can be better combined with the added hydrogel, and the removal effect on environmental hormones is good; and the adopted hydrogel is cross-linked hydrogel based on polyamino acid molecules, has the characteristics of high mechanical strength and good binding degree with poly dimethyl diallyl ammonium chloride, and has a good removal effect on environmental hormones.