The invention discloses a synthesis method of moxifloxacin hydrochloride impurity shown in a formula (I). The synthesis method comprises the following steps of: (a) weighting moxifloxacin or moxifloxacin hydrochloride, methanoic acid and formaldehyde in a molar ratio of 1: (45-145): (20-77), putting in a reflux device, carrying out heating reflux till completely reacting, concentrating reaction liquid to be dry, washing with water, drying, dissolving obtained residues with water, then carrying out decoloring treatment, adjusting the pH value of obtained filtrate to neutral, cooling, crystalizing, separating out crystals, and drying, thus obtaining a midbody; and (b) dissolving the midbody by using an organic solvent, adjusting the pH value of an obtained solution to 1-4 by using hydrochloric acid, then carrying out decoloring treatment, adding water to obtained filtrate, cooling, crystalizing, separating out crystals, and drying, thus obtaining the compound shown in the formula (I). The synthesis method is simple and easy to operate; and the obtained product has high purity (above 99.5%) and can be directly used as an impurity comparison substance.