Digesting pLXSN-mB7-1 by enzyme XhoI and BamHI to gain the opened plasmid pLXSN-mB7-1 with ambly end and BamHI sticking end; Inserting IRES-mB7-2 gene fragment into it to gain the double expressing vector pLXSN-mB7-1-IRES-mB7-2;