The invention discloses two endogenous plasmids of a strain of the genus Salmonella and its origin, on the basis of this kind of plasmid, so the derived plasmid is constructed. The Salmonoccus sp., strain 408 is deposited under the accession number CGMCC No. 13032, which has the characteristics of high salinity and high COD, which makes the application of salmonococcus not only in the field of microbial production, but also has potential application value in the field of high salt and high COD wastewater treatment. The plasmid pS408- 1 and pS408- 2 and its derived plasmids can be autonomously replicated in Salmonella marneys. By means of this plasmid, the foreign gene can be successfully expressed in Brassica. The plasmids also provide a plasmid-depleted Salmonella as a host and successfully expresse lactic acid dehydrogenase derived from Staphylococcus aureus.