The invention discloses a method for synthesizing 1, 3-dibenzyl-4, 5-cis-dicarboxy-2-imidazolone. The prior method uses a Jones reagent to perform oxidation so as to obtain the product, but the Jones reagent is a high-pollution heavy metal strong acidic compound which is gradually limited to be used in the field of pharmaceutical chemical industry because the chromium pollution is serious, and the mass industrial production cannot be realized. The method comprises the following steps: using ethyl acetate as a solvent, adding Na2CO3 into the solvent at room temperature, and then dripping NaClO2 aqueous solution in to the mixture to oxidize 1, 3-dibenzyl-5-formacyl-4-alkoxycarbonyl-imidazoline-2-ketone into the 1, 3-dibenzyl-4, 5-cis-dicarboxy-2-imidazolone. The method uses the NaClO2 for oxidization, which has higher yield than the Jones reagent oxidation and is environment-friendly so as to achieve mass industrial production.