Abstract: Provided are Enterococcus lactis, a drug for preventing or treating tumor, and use thereof, relating to the technical field of biomedicines. Novel Enterococcus lactis is separated and chosen, and the Enterococcus lactis has an important influence on the effect of tumor immunotherapy for cancer patients. Gut microorganisms and immune cells can interact with each other to jointly regulate the human immune system. The provided Enterococcus lactis can induce the differentiation and maturation of iDC (immature DC) cells, and mature DC can effectively activate T cells and increase the infiltration of tumor infiltrating lymphocytes having anti-tumor activity, thereby killing tumor cells. In addition, the provided Enterococcus lactis can induce the differentiation of M0 type macrophages into M1 or M2 type cells.