Abstract: The present invention relates to a recombinant severe acute respiratory syndrome coronavirus (SARS-CoV) non-structural protein (nsp) 12 with an RNA polymerase activity, its expression vector, its preparation method, and its use. According to the present invention, a soluble recombinant SARS-CoV nsp12 with an RdRp activity of initiating SARS-CoV genome synthesis can be over-expressed in the transformed host cells, and conveniently purified with high purity. An in vitro replication system important for studying SARS-CoV replication can be established with the purified recombinant SARS-CoV nsp12. SARS-CoV nsp12 produced by the present invention can also be used as a target for the development of anti-viral agents against SARS-CoV. In addition, materials inhibiting RNA-dependent RNA polymerase (RdRp) activity of nsp12 can be screened efficiently according to the present invention as the optimal conditions for the RdRp assay with SARS-CoV nsp12 were found.