The invention relates to the medical technology field, and aims to solve the problem with traditional tumor curing methods such as radiation therapy, chemical therapy, biological factor treatment, etc., which lack targeting effect, and the problem with immunotoxin treatment such as the nonspecific toxicity of toxin protein and the sensitivity of immunogenicity. The invention discloses targeting nanometer grains that cover toxin protein and are decorated by humanized monoclonal antibody Trastuzumab. The surface of the nanometer grains can be in connection with Trastuzumab, and can be combined with the HER2 antigen on the surface of tumor cells for cell annexation; the toxin protein is covered inside the nanometer grains, and can reduce the nonspecific toxicity of toxin protein and the sensitivity of immunogenicity. The invention further provides a method for preparing the nanometer grains and an application thereof, and has passed the killing extracorporeal tumor cell experiment and the vivo anti-tumor experiment, the results of which show that the biological targeting effect is great, the effect of killing extracorporeal tumor cells is obvious, and at the same time, the grains have little toxicity to normal cells. The vivo experiment further proves that the anti-tumor effect of the targeting nanometer grains is obvious.