Objective An epidemiological investigation and analysis of a food poisoning incident at a school in Pudong New Area was conducted to identify the pathogenic factors, pathogenic food and the source of contamination, and to taken effective control measures and provided reference for the disposal of similar incidents in future. Methods A descriptive epidemiological method was used to analyze the epidemiological characteristics of the incident; a case-control study was used to analyze the suspicious food; interview and on-the-spot investigation were used to conduct food hygiene investigation to understand the process of food production, transportation, storage and raw material sources; samples of anal swabs, food, processing links and drinking water were collected for laboratory testing. Results Fifty-one suspected cases were found, and the clinical manifestations were mainly diarrhea, fever, abdominal pain, headache, nausea and vomiting. The incubation period of onset ranged from 2.5 to 5.1 h, with a median of 20 h. The case-control analysis found that the sandwich at breakfast on September 4^th, 2018 was the suspect high-risk food. Salmonella was detected in the samples of anal swab from 7 cases. Molecular typing analysis of the positive strains of Salmonella by pulsed field gel electrophoresis indicated that they were from the same source of infection, and there was a situation of simultaneous infection of multiple types of Salmonella. Conclusion This was an incident of food poisoning caused by Salmonella, and the suspect food was sandwich. It is suggested that school catering units should strengthen the examination of food supply, strictly implement the sample retention system and implement various food safety measures.