Objective In 2017,a large number of poultry deaths occurred in several poultry markets, and H7N9avian influenza virus was isolated from the dead chickens. This study will track the tendency of the pathogenicity of H7N9virus and analyze the possible pathogenic mechanism. Methods The pathogenicity of the A/ Guangdong/ Th005/2017(Th005) and A/ Anhui/1/2013(Anhui) viruses were compared with the intravenous pathogenicity index (IntravenousPathogenic Index, IVPI) test in chickens. The body temperatures, body weights, and clinical symptoms of the chickens inthe infection groups were continuously measured post-infection. Meanwhile, pharyngeal swabs and cloacal swabs werecollected for viral titer determinations. Results Compared with the Anhui strain, the pathogenicity of Th005 wassignificantly increased in chickens. The IVPI score of Th005 reached 3 in avian. Th005 infected chickens caused asignificant decrease in body weight, body temperature increased above 42℃, and all died. However, the Anhui straincaused no obvious symptoms in chickens. The virulence of Anhui strain was further increased after circulated in chickens.Conclusions Th005 exhibits a high pathogenicity to poultry which may impair poultry industry and increase the existenceof viruses in the environment. Surveillance for the H7N9 influenza viruses, such as Th005, which had risen the virulence,should be stepped up.