Objective: Investigation of the transmission characteristics and risk factors of a human adenovirus outbreak with acute respiratory infection in a primary school of Zhongshan city. Methods: Case definition was carrying out for searching cases. Field epidemiology investigation was taken for information of pathogenesis and risk factors of cases. Results: 32 cases were found including 11 exact cases who were verified that adenovirus nucleic was positive by PCR tests. All cases were 8 to 9 years old and clustered in the same classroom so that the attack rate of this outbreak was 62.7%. The mainly clinical symptoms included fever(100%), pharyngalgia (62.5%), cough (59.4%), headache (31.3%), vomit (28.1%) and rhinorrhea (25.0%). The index case was attacked on May 20, 2019, and went to school on the 20th and 21st. During this periond, the weather was hot and humid. The doors and windows of the classroom were closed and airconditioningwas used to cool down. It was inferred that 22 secondary cases and 9 third generation cases were occurred referring to potential days of adenovirus and holidays. Conclusion: Without isolation of the index case in time and poor classroom ventilation may be the main risk factors for the spread of this epidemic.