Objective To study the outbreak rules of influenza A virus and the evolution of hemagglutinin(HA). Methods A phylogenetic tree of HA gene was constructed respectively for influenza A virus in China mainland and Hong Kong District of China to analyzed the influenza genome data published in China mainland and compare the HA gene sequences in humans, avian, and mammals in China mainland and Hong Kong District of China. The selection pressure of HA gene from different hosts were analyzed. Results The analysis o...