Fresh mackerel were stored in the refrigerated sea water system. The bacterial composition and changes during storage were studied by selective medium and 16S rDNA sequence analysis. Moreover， the dominant spoilage bacteria in the spoilage samples were identified and the specific spoilage bacteria were determined by the potential spoilage ability tests. The results showed that the dominant bacteria changed from Psychrobacter sp. to Pseudomonas sp. and then to Shewanella sp. during the storage. Shewanella sp.， Enterobacter sp. and Citrobacterfreundii were the dominant bacteria in the spoilage samples. The results of the potential spoilage ability tests indicated Shewanella sp. made more contribution to the increase of TVB-N， TMA and TVC while Citrobacterfreundii contributed more to the accumulation of histamine. Yet， TVC in the samples incubated with Citrobacterfreundii were kept at low level. Therefore， Shewanella sp. was identified as the specific spoilage bacterium of mackerel stored in the refrigerated sea water system.