Provided are a transformant used for losing weight and reducing fat, and a construction method for the transformant. The transformant is a recombinant lactococcus lactis obtained by replacing a lactococcus lactis thymidylate synthase gene with a codon optimised human oxyntomodulin gene via homologous recombination. Also provided is an application of the transformant in the preparation of a blood glucose-reducing drug, a fat-reducing drug and a food.