The purpose of this study was to understand the geographic extent, risk factors, and attack rate of the 2009 pandemic influenza A (H1N1) virus [A(H1N1)pdm09] infection in Shandong Province, China and to identify the influencing factors. A randomized serological survey of A(H1N1)pdm09 infection was carried out in August and September 2010. A total of 4,549 participants involved in the survey had their antibody levels tested by hemagglutination-inhibition assay. The overall seropositive rate for A(H1N1)pdm09 antibodies was 25.85%. The seropositive rate was 25.89% for the unvaccinated group, with statistically significant differences among individuals of different age groups, occupations, and cities. The highest seropositive rate was observed in young children aged 0-5 years and elderly people aged ≥60 years. Multivariate logistic regression revealed that subjects in rural areas had significantly higher odds ratio of A(H1N1)pdm09 seropositivity than those in the capital city. Individuals belonging to all professions, except for teachers, had significantly lower odds ratio of A(H1N1)pdm09 seropositivity compared with children in family care. Our data indicated that almost 26% of the residents in Shandong Province had appropriate antibody titers against A(H1N1)pdm09. This seroepidemiology study provides valuable data for understanding the epidemiology of the 2009 pandemic influenza and for planning future intervention strategies; moreover, it highlights the significance of seroprevelance studies.