ObjectiveTo investigate the causes of an outbreak of Serratia marcescens (S. marcescens) catheterrelated bloodstream infection(CRBSI) in a breast surgery department，and provide  basis for the prevention and control of healthcareassociated infection(HAI). MethodsEpidemiological investigation was performed by analyzing clinical data and sampling. ResultsNine cases of CRBSI occurred in a breast surgery department  on January 2—8,2014. Four  patients isolated S. marcescens from their catheter tips and blood culture specimens, and 5 patients only isolated S. marcescens from their catheter tips.The resistant spectrum of S. marcescens were basically the same, but cefoxitin susceptibility results were different, 7 strains were  intermediate, 1 resistant, and 1 sensitive, antimicrobial susceptibility testing results of S. marcescens from catheter tips and blood culture specimens of the identical patients were the same. S. marcescens were detected from the hands of medical staff(n=1)，door handle of the therapeutic room (n=1), and surface of the treatment trolley (n=1), the resistant spectrum were consistent with that of bacteria from blood culture. All infected patients were effectively controlled after catheter removal and antimicrobial use. Wards were not detected S. marcescens and no new infection occurred again after strict disinfection. ConclusionThe main causes of the outbreak are contamination of environment of breast surgery department，nonstandardized hand hygiene，long time catheterization. Medical staff should strengthen hand hygiene compliance，pay attention to disinfection of environment and surface，effectively prevent and control HAI.