A method for the quantitative synthesis of prasugrel-degrading impurity dione hydrochloride, namely 1-cyclopropyl-2-(2-fluorophenyl)ethanone, as a starting material, selecting a suitable oxidizing agent, and oxidizing the carbon atom adjacent to the keto carbonyl group of 1-cyclopropyl-2-(2-fluorophenyl)ethanone in a reaction solvent to obtain prasugrel-degrading impurity dione hydrochloride represented by the formula I in one step, namely 1-cyclopropyl-2-(2-fluoro- phenyl)-1,2-ethanedione (pragre dione),
