The invention relates to the technical field of fuel additives, and particularly relates to an additive for reducing reduced fuel oil pollutant emission and a preparation method thereof. The additive is prepared by combining polyoxymethylenedimethyl ether, formaldehyde diethyl acetal, high-boiling-point biodiesel, coal tar hydrogenated fuel oil, octanol and a mixture of organic rare earth lanthanum and isocaprylic formate, so that the additive is a yellow transparent viscous liquid, and has a viscosity of 8.0-10.0mm<2>/s at 30 DEG C and a moisture mass percentage content of 0.5-1.0 percent; and when the additive is added into reduced fuel oil, the total amount of particle emission can be effectively reduced, and the quality of reduced fuel oil can be effectively improved.