Abstract: The specific serum IgG was purified through DEAE-sephadex-A50 low pressure chromatography system after rude purification. This IgG was used as target molecular to screen the phage display 12-mer random peptide library in different conditions for 4 rounds. By altering the condition of screening, the yield ratio increased from 1.44×10^-6 to 8.9×10^-5, and the P/N value increased gradually, which meant that specific enrichment had been achieved.5 phage clones were identified as positive by ELISA and they did not have cross reactions with irrespective antibodies. ssDNA of 5 positive clones were purified for DNA sequencing and the sequences of peptides displayed on the positive clones were analyzed. 4 continuous amino acid (RVDV) of 3 clones were identical, and they were homologous to the original peptide of Actinobacillus pleuropneumoniae RTX toxin Ⅰ.