The invention discloses a preparation method of poly (
diallyldimethylammonium chloride) as flocculating agent for treating
sewage. The preparation method comprises the following steps: (1) adding an initiator, a complexing agent and
ethylene glycol as a
chain transfer agent in an
aqueous solution of
diallyldimethylammonium chloride, stirring uniformly, and feeding
nitrogen to remove air in a reaction container; (2) heating reaction liquid obtained from the step (1) while stirring, and carrying out
polymerization reaction; (3) heating to the temperature of 65-85 DEG C, adding the initiator in the reaction liquid again, and adding water in the reaction liquid for reaction; and (4) under protection of the
nitrogen, carrying out insulation and curing to obtain the poly (
diallyldimethylammonium chloride). The method is simple in process and easy to operate, the
reaction temperature is convenient to control,
polymerization reaction is stably carried out at
low speed, products with medium and low relative molecular
mass are easily obtained, and the preparation method is suitable for treatment of oilfield chemical flooding output liquid. The prepared flocculating agent for treating the
sewage is high in oil removal rate, high in
monomer conversion rate, good in water purification effect, low in
viscosity of products and convenient to inject, and has universal applicability.