The disclosure relates to the field of medical technology, and provides use of an agent for inhibiting the ectopic expression of a PIWIL2 gene and a protein antibody against the ectopic expression of the PIWIL2 gene in preparation of an anti-tumor drug. The disclosure also provides an anti-tumor pharmaceutical composition and a method for treating a tumor. It has been experimentally proved that, treating human or mouse tumor cells with KAO3 upon inoculation can effectively inhibit the tumorigenesis in a mouse. Furthermore, injecting KAO3 into established lymphoma, breast cancer, lung cancer and cervical cancer can significantly inhibit tumor growth and prolong the survival of a tumor-bearing mouse. The inhibitory effect of KAO3 is associated with KAO3-specific antigens expressed on the surface of a tumor cell. KAO3 induces apoptosis of the tumor cell by blocking the cell cycle in a G2/M phase, inhibiting DNA synthesis and activating a complement. Therefore, the anti-PL2L60 mAb (KAO3) is a potential therapeutic candidate drug for treating a cancer.