The invention discloses a carboxyl-terminal specific anti-human amyloid protein monoclonal antibody gene and its encoded polypeptide and an application, the gene comprises a heavy chain variable region gene and a light chain variable region gene, wherein a sequence of the heavy chain variable region is showed as SEQ ID NO:1, and a sequence of the light chain variable region is showed as SEQ ID NO:3. According to the gene provided by the invention, because a Abeta 42 oligomer can be specifically recognized, and Abeta 40 and Abeta 42 oligomers which have the difference of two amino acid can be distinguished, in an immunotherapy experiment, the combination of monoclonal antibody gene and Abeta 40 can be avoided, thereby the side reaction caused by the non specific recognition can be reduced; the polypeptide obtained according to the gene coding is amyloid protein carboxyl-terminal polypeptide, the immunization effect is good, the mice immunization serum has high valence, and its antigen recognition capability has conformational characteristics, the invention can be used for AD early stage diagnosis.