The invention discloses a preparation method of an oxidizing polyethylene dispersoid sewage treating agent. The preparation method comprises the following steps: step 1, sequentially adding oxidizing polyethylene, paraffin and rosin to a reactor, heating and stirring uniformly; step 2, stirring a reaction liquor and adjusting pH value, so that the pH equals to 10; step 3, stirring the reaction liquor for 0.5-1.5 hours, further stirring and slowly adding boiling water until the liquor is brown and transparent, further adding the boiling water while stirring until the solid content is 18% and the reaction liquor is gray; step 4, cooling to room temperature while stirring, wherein the reaction liquor is grey white, and pH equals to 8-10, thus preparing the oxidizing polyethylene dispersoid sewage treating agent. The obtained oxidizing polyethylene dispersoid sewage treating agent can be used for adsorbing organic matters and heavy metal in sewage, thus achieving the purpose of greatly reducing COD in water and the concentration of the heavy metal.