The invention discloses antibacterial and antitumor application of exosomes secreted by NK cells (aNK) activated by transmembrane IL-21 and multiple miRNAs contained in the exosomes, and provides a new method for bacterial infection and antitumor treatment. The method has the advantages that 1, the method is different from antibiotic treatment, and exosome antibiosis has lower side effects; 2, effects of antibacterial treatment can be enhanced through joint use; 3, the exosomes can be stored at low temperature, the miRNAs can be artificially synthesized, costs are low and large-scale production can be achieved; 4, the exosomes and the multiple miRNAs have inhibitory effects on a variety of bacteria, and have wider antibacterial spectra; 5, the miRNAs act on a variety of shared signal pathways of tumor cells, can inhibit a variety of tumors, and have a wide range of antitumor application.