Abstract: A polypeptide magnetic nanoparticle, comprising: a specific targeting polypeptide and a magnetic nanoparticle. The amino acid sequence of the specific targeting polypeptide is VRRDAPRFSMQGLDA-X, and the C-terminal X thereof is a sequence of 5 to 20 amino acids. The amino acid in the amino acid sequence of C-terminal X is selected from one or more of: C, G, and N. The polypeptide magnetic nanoparticle can be used for CTC detection and molecular typing of various cancer types, including esophageal cancer, liver cancer, lung cancer, stomach cancer, bladder cancer, skin cancer, melanoma, breast cancer, colorectal cancer, cervical cancer, etc. Biomarkers of CTC molecular typing include PD-L1, HER2, ER, PR, AR, EGFR, VEGFR, CXCR4, etc. The polypeptide magnetic nanoparticle is helpful for CTC detection and the qualitative and semiquantitative analysis of expression levels of biomarkers at CTC level, thereby performing precise therapy such as targeted individualized targeted therapy or immunotherapy on patients.