The invention discloses a preparation method and application of a plastic toughening dispersant. Graphene oxide and carboxyl-terminated hyperbranched polyester are adopted as main raw materials to prepare the plastic toughening dispersant; relatively active reactivity and interface activity function of graphene oxide are utilized, and the carboxyl-terminated hyperbranched polyester with high activity, low viscosity and good dispersity is adopted for grafting to modify surface of graphene oxide, so that graphene oxide has better organic solvent dissolution dispersity; meanwhile, a hydrogen bond and a plastic composite material form a chemical bond to anchor and bridge plastic resin and each component thereof, so that mechanical performance and other performances of the composite material are greatly improved. The device and the method used for preparing the plastic toughening dispersant are simple, low in cost and easy for large-scale popularization; the prepared plastic toughening dispersant is small in additive amount, a modifying method is convenient, and effect of improving mechanical performance, processing performance, thermal performance, and the like of the plastic composite materials is relatively obvious.