Abstract: Objective To analyze the role of populations of different age in the influenza epidemics in Jiaxing city between 2009 and 2011.Methods Seven-day moving averages method was used to find the epidemic wave of influenza-like illness;the relationship between the epidemic wave and influenza virus detection rate was analyzed and the median and interquartile range of epidemic curve was calculated to estimate the role of different age groups in the influenza epidemic session.Results The correlation coefficient between weekly influenza-like cases and influenza virus positive rate was 0.46(95% confidence interval95%CI:0.29-0.60) and the three in four epidemic waves showed around 50% influenza positive rate.The young age group and the populations at the age of 5-24 years were the leading groups during seasonal and pH1N1 epidemic,respectively.Conclusion Influenza-like illness epidemic is closely related with influenza-positive detection rate;during the seasonal influenza epidemics,small age group is the key population,while for new influenza epidemic the leading transmission population is the control target.