Abstract: Objective To understand the infection status of influenza A(H1N1)2009 virus and its epidemic trend in the post-peak phase of the pandemic among the population in Hainan province,2009. Methods Serum specimens were collected from outpatients in hospitals and volunteers in blood donation center between December 2009 and January to March 2010 in Haikou city,and tested with hemagglutination-inhibition(HI)assays.Rates of protective antibody against influenza A(H1N1)2009 were estimated. Results For the five batches samples,the rates of protective antibody against influenza A (H1N1)2009 were 10.50%,16.06%,31.35%,23.16%,and 38.02%,respectively,showing a rising tendency with time (linear-by-linear association,χ²=81.17,P<0.05).Except the age group of 0-5 years,the antibody positive rates of 6-17,18-55,and≥56 years age groups increased with the time(χ²=4.22,P>0.05;χ²=21.25,P<0.05;χ²=39.05, P<0.05;χ²=26.85,P<0.05).And the age group of 6-17 years had the highest positive rate among the four age groups. Conclusion The influenza A(H1N1)2009 epidemic reached its peak in Hainan province during October to November 2009.After the peak,the antibody level against influenza A(H1N1)2009 increased gradually.Till March 2010,an immune barrier of population had formed initially,indicating that the epidemic might maintain at a low level for a certain period of time.People aged 6-17 years had the highest infection rate among all age groups.