The invention discloses a method for synthesizing 5-acetylacetamidobenzimidazolone. The method comprises the following steps: condensation, namely putting o-phenylenediamine, urea and chlorobenzene into a reaction container according to a proportion, preserving heat at 95-130 DEG C for 12-14 hours, absorbing ammonia with water in the reacting process, filtering and washing after the reaction is completed, and baking the prepared material at 100-120 DEG C to prepare benzimidazolone; nitrification, namely putting the benzimidazolone, water and concentrated nitric acid into the reaction container according to a proportion, preserving heat for 3-4 hours, filtering and washing after the reaction is completed, and baking the prepared material at 100-120 DEG C to prepare the 5-nitro-benzimidazolone; and carrying out reduction and acetylation. By using the method, the product purity can be over 99.5 percent, and the product quality reaches and exceeds the quality index (99.2 percent) of similar products overseas. Particularly, catalyst hydrogenation reduction is adopted to replace iron powder reduction, so that generation of massive wastewater and iron sludge can be avoided, and the production efficiency and product yield can be improved.