The invention belongs to the technical field of oilfield sewage pretreatment, and particularly relates to a sterilizing agent used for oilfield polymer-contained sewage and a preparation method of the sterilizing agent. The sterilizing agent is prepared in a mode that benzyl halide is reacted with polyethylene polyamine, wherein the molar ratio of the benzyl halide to the polyethylene polyamine is 1:(2.0-2.2), and the preferred ratio is 1:2.0. The benzyl halide is benzyl chloride or benzyl bromide, and the polyethylene polyamine is one of diethylenetriamine, triethylene tetramine and tetraethylenepentamine. According to the sterilizing agent, positive charges in the molecular structure of the sterilizing agent are dispersed, the compatibility of the sterilizing agent with the polymer-contained sewage is good, and the phenomenon that suspended solids in the sewage are not increased cannot be caused; due to the sterilizing agent molecule with lipophilic groups at the two ends and hydrophilic groups in the middle, the permeability of the molecule is good; meanwhile, the raw materials of the sterilizing agent are easy to obtain, the synthetic method is simple, the cost is low, the sterilizing effect is good, and the sterilizing rate is higher than 98.0%. Therefore, the sterilizing agent can be widely used in the sterilization process of the oilfield polymer-contained sewage.