The present disclosure provides a preparation method for metformin hydrochloride. The preparation method includes: microwave heating raw materials containing dicyandiamide and dimethylamine hydrochloride, and reacting the two at 100° C. to 160° C. to obtain a product system containing metformin hydrochloride. Microwave heating is adopted to heat the dicyandiamide and the dimethylamine hydrochloride. Compared with a conventional heating mode that requires heat to be gradually transferred from the outside to the inside, microwave heating can directly heat each part inside the reactant, which can make the internal temperature of the reactant more uniform, thereby reducing the generation of impurities.