The DNA fragment encoding the nucleocapsid protein (N) of PRRSV BJ4 strain were cloned into the BamH I / EcoR I sites of pET28a vector to construct the expression plasmid pET28-N by designing special primers. The soluble protein (P28-N) were obtained by introducing the expression plasmid into E. coli BL21 (DE3) host cell, and the amount of recombinant protein reached to 28% of the total mass of bacterial protein. PET28-N were purified by nickel-affinity column of Proband resin. The circular dichroism (CD) analysis showed that the purified PET28-N shared a significant (26%) alpha-helical structure, beta-sheet (23.7%), beta-turn (19.8%), and random coil (30.3%), respectively. Finally,the secondary structure of N protein of PRRSV was deduced.