Abstract: 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 of 1 252 and 485 HA genes selected from China mainland and Hong Kong District showed that the outbreak times and frequencies of influenza A virus had been more than their historical records since 2002. The phylogenetic trees of HA gene for China mainland and Hong Kong District showed that their topological structure was identical. The selection pressure of HA gene was greater from humans than from avian. Conclusion The selection pressure of HA gene is different from different hosts, suggesting that the rapid evolution of HA gene may pose a potential threat to the health of mankind.