The cDNA fragment encoding the truncated GP5 and the full-length M protein of Porcine Reproductive and Respiratory Syndrone Virus (PRRSV) were orderly fused to the downstream of glutathione S-transferase (GST) of pGEX-KG expression vector, resulting in the fusion expression plasmid pKG-56. After transformed into E. coli BL21 (DE3) and induced by IPTG, the results of SDS-PAGE showed that the GST-GP5-M fusion protein was expressed in high level. Western-blot was performed to confirm that the expressed fusion protein could specifically react with antiserum against PRRSV. The fusion protein was further purified and used as an antigen to establish a novel PRRSV ELISA diagnose assay (P56-ELISA). Comparison between P56-ELISA and the abroad kit IDEXX-ELISA showed the two methods had 94.1 percent agreement by detecting 205 serum samples, indicating that the indirect P56-ELISA was specific and sensitive. The correlation between virus neutralization antibody of the infected pigs (not convalescent pigs) and antibody response to the fusion protein GP5-M was further studied. The regression function analysis suggested that there was no significant correlation between ELISA antibody response (OD630 nm) to the fusion protein GP5-M in clinical serum and their specific neutralizing titers.