The invention discloses a preparation method of a rubber plasticizing agent dioctyl sebacate. The method includes the following steps: 1, adding sebacic acid, 2-ethyl hexanol and a catalyst stannous oxide into a four-neck flask equipped with a thermometer, a reflux distributor and an agitator, introducing nitrogen, and conducting esterification reaction under 200 DEG C-230 DEG C; 2, after the reaction, cooling to 90 DEG C, adding lye water for neutralization, washing, standing for layering, releasing lye water, conducting reduced pressure distillation, and finally add activated carbon for adsorption and filleting to obtain the dioctyl sebacate product. The preparation method provided by the invention employs a non-solid acid catalyst stannous oxide for catalytic synthesis of dioctyl sebacate, and has the advantages of high catalytic activity, short reaction time, small amount of catalyst, light color of the obtained product, simple post treatment process, no corrosion of the catalyst on the equipment, no waste liquid produced, green, and environment-friendliness.