Abstract: Objective The study was conducted to investigate the cause and characteristics of three cases of influenza (flu) outbreak in Xi'an in order to further the prevention and control of influenza. Methods Epidemiological investigation was performed among different groups of people to which the influenza infected patients belonged. Throat swab samples were collected for influenza virus isolation. Results The attack rate of three schools was 23.2%; the age of onset concentrated mainly in 10 to 12. A total of 19 strains of B-type influenza virus were isolated from 43 throat swabs. Conclusion All three cases were caused by the influenza B virus. Therefore, it is of great significance to strengthen the etiological surveillance for the prevention of influenza outbreaks.