Objective: To investigate the epidemiological data of an outbreak of influenza A in a primary school in Shanghai, to provide reference for targeted prevention and control measures. Methods: A field epidemiological method was used to investigate and collect the data of influenza A outbreak in a primary school in Jing an District from November 8 to December 6, 2022, through on site follow up and telephone return visit to health teachers and cases. The distribution characteristics of the epidemic were analyzed by descriptive epidemiology. Results: The first case developed symptoms of cough, sore throat, and fever on the morning of November 8th 2022, with a maximum body temperature of 38.6 ℃. Later, the clinical diagnosis was influenza A. The total number of influenza like cases reported in this outbreak is 99, including 92 students and 7 teachers. The total incidence rate was 9.45%. The clinical symptoms of all cases were fever, sore throat, and cough; 27 cases of influenza A were diagnosed by hospital rapid diagnostic reagents. The second grade students had the highest case incidence rate(24.46%), and there was a statistically significant difference in case incidence rates among students of different grades ( χ 2=48.28, P ＜0.01). The case incidence rate on the second floor was the highest (23.47%), and there was a statistically significant difference between the case incidence rates on different floors ( χ 2=52.38, P ＜ 0.01 ). Etiological testing showed that the influenza virus causing this outbreak was type A H3N2 virus. Conclusion This outbreak is a campus cluster outbreak caused by influenza A (H3N2) virus. The health and education departments should strengthen cooperation to effectively implement prevention and control measures of infectious diseases, and timely identify the source of infection and cut off the transmission route.