Salmonella typhimurium 17Y, isolated from one diseased pig that was clinically diagnosed as pig salmonellosis, was a multiresistance strain with resistance to 14 antibiotics among tested 19 antibiotics. In this study, the resistance to 11 antimicrobials was reversed by high temperature and high concentration (0.5%) of SDS, resulting in the sensitive strain 17S1. PCR results showed that the resistant genes BlaTEM, blaOXA-1, cat 1, tet (B), aacC2 located on the plasmid. Furthermore, PCR detected the class I integron which carries dhfrX II for trimethoprim resistance, aadA18b for aminoglycoside resistance and sull for sulfamethoxazole resistance. The integron was identified to exist in the plasmid. Because the target genes gyrA and parC for quinolone category were detected by PCR from both resistant and sensitive strains, it was determined that the genes gyrA and parC were located in the bacterial genome. The gene sequencing of gyrA and parC revealed that a point mutation AAC --> GAC resulting in one amino acid replacement of N87D in gyrA occurred for the sensitive strain 17S1. It was demonstrated that the amino acid 87 was a hot point for mutation in quinolone resistance determining region (QRDR). The finding suggests that the amino acid replacement of N87D is responsible for the quinolone susceptibility. In addition, the 100 continuous passages of the sensitive strain showed that the drug sensitive status was stable. However, when the drug pressure maintained for a long time, the resistance was induced again. Meanwhile, 6 salmonella plasmid virulence genes (spvA-D, R and rck) were eliminated with the resistance reversal, indicating that the virulence plasmid was cured. Reasonably, the bacterial virulence decreased shown by 10- fold increase of LD50 for the sensitive strain, and the statistical significant decline of in vivo spread and growth (P < 0.05) in mice. Taken altogether, the multidrug resistance of Salmonella typhimurium was determined by its plasmid. The plasmid elimination with SDS reversed most of the resistance (11/14) and decreased the bacterial virulence. Therefore, strategy to eliminate the plasmids would be an effective way to deal with the multiresistance issue. However, drug control in routine clinical practice would not be neglected at any time.