Objective To analyze the characteristics and causes of an outbreak of hepatitis A in a middle school and provide scientific basis for epidemic prevention and control. Methods The epidemic situation of the drinking water in the school was analyzed with the method of field epidemiological investigation, combined with the clinical and laboratory test results, and the reasons for the outbreak were analyzed. Results A total of 49 cases of hepatitis A were found in a middle school, with attack rate 1.12% and all the patients were grade three high school students. There were 46 resident students and 3 non-resident students. The peak period was 17-19 February, accounting for 32.65% of the total number of diseases. The main clinical manifestations were fever, Nr, sclera (skin) yellow staining, urine yellow, nausea and vomiting. There were no severe or fatal cases. Drinking direct drinking outside the dormitories and classrooms was a risk factor for hepatitis A. We collected 4,494 blood samples, of which 49 of hepatitis A IgM positive, and 124 of abnormal liver function. We tested 32 water samples and results indicated that water from 1st floor piping end of the office building and sewage drain outlet were positive in hepatitis viral nucleic acid. The well water was out of limits in total bacterial count, total coliform group, and heat-resisting coliform. Conclusion The incident was a hepatitis A outbreak.The main reason was the drinking water contaminated by pathogens at the school.