The invention belongs to the field of drug synthesis and particularly relates to a method for preparing bendamustine hydrochloride. According to the preparation method, concentrated hydrochloric acidis adopted as a refining solvent, thus, the bendamustine hydrochloride can be rapidly dissolved in the concentrated hydrochloric acid at the temperature of 20 DEG C to 45 DEG C, and the production cycle is short; the method is applicable to enlarged-scale industrial production, the refining efficiency is high, the yield is high, the reproducibility is good, and the obtained bendamustine hydrochloride is high in stability; and proven by high-temperature, high-humidity, accelerated and long-term tests, related substances and content thereof are free of obvious changes, the content of each singleimpurity is lower than 0.1%, and particularly, an impurity, i.e., a bendamustine dimer is undetectable, so that the bendamustine hydrochloride meets the technical requirements of ICH European Union (EU) quality research technique governing principles on limit of impurities.