Objective To provide the scientific basis for prevention of such incidents， a foodborne disease outbreak in school caused by Salmonella enteritidis was investigated and the risk factors was traced.Methods Descriptive epidemiological methods were used to analyze the characteristics of the incident. Case-control studies were used to investigate the suspected food. Environmental hygiene investigations were used to trace the process of food contamination， and pathogen isolation， serotyping analysis， and pulsed-field gel electrophoresis （PFGE） were used to detect and analyze the Salmonella enteritidis isolated from the patients， food and environmental samples.Results A total of 71 patients were reported in this outbreak， and the epidemic curves were consistent with a sustained homologous outbreak pattern. The case-control study， environmental health survey and etiology study suggested that sandwiches sold in the school supermarket were the etiological food for this incident （OR=302.09， 95%CI=75.18~1 213.97）. Salmonella isolates from 8 samples （12.70%， 8/63） collected from patients， food and environmental were detected， and their PFGE profile were identical， confirming that the salad dressing used in the cause-food sandwich was contaminated with Salmonella during the preparation process， which led to this foodborne illness outbreak.Conclusion This outbreak was caused by sandwiches contaminated by Salmonella enteritidis， and the sandwiches were contaminated by homemade salad dressing. It is suggested that all foods containing raw egg ingredients that have not been adequately heat-treated should be banned from schools， and that the relevant administrative departments should strengthen sanitary supervision of such foods.