Abstract: Objective To understand the infection status of influenza A (H1N1) virus and the epidemic pattern of influenza A (H1N1) in Ji'nan by the serum antibody detection in the selected population in Ji'nan and provide scientific evidence for influenza pandemic response. Methods The questionnaire survey and blood sample collection were conducted in the people in 5 age groups sleeted in 3 urban districts by stratified random sampling and hemagglutination inhibition test was performed to detect antibodies to influenza A (H1N1) virus. Results Three infection status surveys were conducted in January, March and August in 2010. The antibody positive rate was 34.67%, 32.82% and 34.67% respectively The antibody level to influenza A (H1N1) virus was highest in people aged 5-24 years with the positive rate of40%. The middle school students had the highest antibody level. There was no significant difference on the positive rate between males and females. The antibody level in resident students was higher than non-resident students. There was no significant difference on the positive rate between the people received seasonal influenza vaccination and those didnt received the vaccination, but the positive rate in people who received A (H1N1) vaccine was higher than those didnt received the vaccine. Conclusion Over 30% of the people in urban districts in Ji'nan were infected with influenza A (H1N1) virus, but the infection rate was over 40% in school and child care settings and the infection rate was 20% in people aged 60 years. No second or third epidemic occurred as previously predicted.