The present invention discloses a β-galactosidase mutant, and a preparation method and application thereof, belonging to the fields of gene engineering and enzyme engineering. Amino acids of specific sites in the β-galactosidase are mutated, the β-galactosidase is transferred into a recombinant bacterium, and enzymatic transformation is performed under optimized conditions, so that the yield of galactooligosaccharide produced by the mutant reaches 59.8%, which is increased by about 70% as compared with that of wild enzyme, thereby implementing the increase of the galactooligosaccharide yield. The present invention has very high industrialized application value.