The inhibitory activity of silver nanoparticles and three commercial preservatives was studied against four bacteria isolated from the naturally spoiled fishes. Antimicrobial assay in vitro and the cell membrane assay were used to evaluate the inhibitory capacity and the possible mechanism of silver nanoparticles. The results indicated that silver nanoparticles showed great and broad antimicrobial activity. Silver nanoparticles had great inhibitory activities against all the tested Gram-negative bacteria with the minimum inhibitory concentration of 0.3125-1.250 mg/mL， while three commercial preservatives had no inhibitory activity. Silver nanoparticles could break the cell membrane and cause the release of UV-absorbing substances， such as proteins， and potassium ion in the tested bacteria, with good slow-release property. Silver nanoparticles has a good potential for the application of food preservative.