The present disclosure provides a method for preparing a nicotinamide mononucleotide cocrystal, and aims to solve the technical problems of larger content/weight difference and inconsistent quality of nicotinamide mononucleotide (NMN) medicines or health care products due to poor fluidity of existing nicotinamide mononucleotide crystals. The method includes the steps of mixing nicotinamide mononucleotide as an active pharmaceutical ingredient with isonicotine as a cocrystal former by adopting solution synthesis and then performing crystal precipitation. The method has the advantages of simple operation and wide application range.