The invention relates to a refining agent for synthesizing dioctyl sebacate and a preparation method for the dioctyl sebacate. The method for synthesizing the dioctyl sebacate comprises esterification, washing, dealcholization, refining and filtration, wherein in the refining step, the refining agent is prepared by mixing potassium carbonate, magnesium hydroxide, active carbon and magnesium sulfate in a certain proportion. By the invention, the industrial production is facilitated and the production efficiency is improved; and a high vacuum refining step is saved during production, and the input of high vacuum pump equipment is absent, so the equipment investment is saved, the production cost is reduced and the market competitiveness of the product is improved; the refining agent is prepared by mixing the potassium carbonate, the magnesium hydroxide, the active carbon and the magnesium sulfate in a proportion, the components have the characteristics of low prices and capacity of being easily purchased; meanwhile, the product quality and long-term storage stability are ensured; and the preparation method is easy and convenient to implement, the investment of production equipment is saved and the production efficiency is improved.