Abstract: In order to prepare monoclonal antibody against Cap protein of Porcine circovirus type 2（PCV-2）, the Cap gene was cloned onto the prokaryotic expression vector pET-28a（+） and transformed into Escherichia coli BL21（DE3） receptor cells. Recombinant expression bacterium pET28a-Cap/BL21（DE3） was induced by IPTG to express the recombinant protein, and purified by His label. BALB/c mice were immunized subcutaneously with the purified recombinant protein, and the serum titer of the immunized mice was determined after the immunization, and cell fusion was performed when the serum antibody titer of the mice reached 1∶100 000. Subcloning was performed by limited dilution method; positive hybridoma cell lines capable of stable antibody secretion and high antibody titer were screened; antibody subclasses of hybridoma cell lines were identified; a cell line with the highest antibody titer was selected to be injected into the abdominal cavity of mice again（5×10~5 per cell）, and ascites were taken for monoclonal antibody purification by affinity chromatography. The titer and specificity of monoclonal antibody were determined by indirect ELISA, and the reactivity of monoclonal antibody was detected by Western-blot and indirect immunofluorescence assay（IFA）, respectively. The results showed that the Cap gene with the size of 708 bp was amplified by PCR, and the recombinant expression plasmid pET28a-Cap was successfully constructed after double digestion and sequencing. The recombinant protein expressed by the recombinant expression bacterium pET28a-Cap/BL21（DE3） after induction could react specifically with His labeled antibody, and the purified recombinant protein showed a clear single band at the expected location（31.7 ku）. A total of 8 positive monoclonal cell lines（1E5, 2A9, 3C6, 4B3, 5F7, 6D11, 7A2, 8G1） with high antibody titer and stable antibody secretion were screened, and the antibody subclass was identified as IgG1 subclass. Strain 7A2 had the highest titer and was used for the preparation of monoclonal antibody. Indirect ELISA confirmed that monoclonal antibody of strain 7A2 only reacted with PCV-2 Cap protein, but did not react with Porcine reproductive and respiratory syndrome virus（PRRSV）, Porcine epidemic diarrhea virus（PEDV）, Classical swine fever virus（CSFV）, Porcine parvovirus（PPV） and Porcine pseudorabies virus（PRV）. Both Western blot and indirect immunofluorescence tests confirmed that the PCV-2 Cap protein was specifically recognized by 7A2 monoclonal antibody. The results indicated that PCV-2 Cap monoclonal antibody with high purity, strong specificity and good reactivity was successfully prepared.