Several epidemic influenza viruses leading to worldwide periodical pandemics all result from the genetic reassortment of different influenza viruses. The novel 2009 A/H1N1 virus is a reassortment virus evolved from swine influenza virus A/H1N1,avian influenza virus H5N1,and human influenza virus A/H1N1. The 8 fragmente genes of the novel A/H1N1 virus had their own evolutionary characteristics.All the pandemic viruses in humans originate from avian influenza viruses and are transferred into humans after reassortment processes in pigs. Pigs as middle host and a mixing vessel of influenza A virus play an important role in the evolution of the 2009 novel A/H1N1 virus. More attention should be paid on the role of swine in the prevention and control of novel H1N1 virus epidemics in future.